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Langmuir Isotherm

Environmental Engineering · FE Reference Handbook section

Environmental Engineering
3 formulas
10 exam-style examples
~51 min
All Environmental Engineering lectures

Core formulas for this FE topic

Definitions, applicability, units, assumptions and worked examples for each relation.

Worked exam-style examples

The four ways this section is written on the real exam — thoughts first, then equations, then substitution.

Example 1
Langmuir Isotherm — solve for adsorption capacity (x/m) — Langmuir Isotherm

Langmuir isotherm monolayer adsorption model for carbon treatment Given maximum adsorption capacity (a) = 12.0000 mg/g; Langmuir equilibrium constant (b) = 0.0540 L/mg; equilibrium concentration (C_e) = 44.3000 mg/L, determine the adsorption capacity (x/m) (x_m) in mg/g.

Given

  • maximumadsorptioncapacity(a)=12.0000mg/gmaximum adsorption capacity (a) = 12.0000 mg/g
  • Langmuirequilibriumconstant(b)=0.0540L/mgLangmuir equilibrium constant (b) = 0.0540 L/mg
  • equilibriumconcentration(Ce)=44.3000mg/Lequilibrium concentration (C_e) = 44.3000 mg/L

Find

adsorption capacity (x/m) (x_m), in mg/g

Start with the thinking

  • The governing relation printed in this handbook section is Langmuir Isotherm.
  • Everything except x_m is given, so isolate x_m symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The Langmuir isotherm describes monolayer adsorption capacity and can be expressed in a linearized form for parameter fitting.
1/Ce1/(x/m)Langmuir isotherm linearized form

Figure 1 — schematic for Langmuir Isotherm — solve for adsorption capacity (x/m) — Langmuir Isotherm

Step-by-step solution

  1. Step 1 — State the governing relation:

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}
  2. Step 2 — Rearrange symbolically for x_m:

    xm=abCe1+bCex_{m} = \dfrac{abC_e}{1+bC_e}
  3. Step 3 — List the givens: maximum adsorption capacity (a) = 12.0000 mg/g, Langmuir equilibrium constant (b) = 0.0540 L/mg, equilibrium concentration (C_e) = 44.3000 mg/L.

  4. Step 4 — Substitute the given values:

    xm=abCe1+bCex_{m} = \dfrac{abC_e}{1+bC_e}
  5. Step 5 — Evaluate:

    xm=8.4625 mg/gx_{m} = 8.4625\ \text{mg/g}
  6. Step 6 — Check: returning x_m = 8.4625 mg/g to

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}

    reproduces the given quantities, and both sides carry the same units.

Answer:
xm=8.4625 mg/gx_{m} = 8.4625\ \text{mg/g}

Why the other options are there

  • 16.9249 — kept a factor of two that cancels in the correct rearrangement.
  • 4.2312 — dropped that same factor in the other direction.
  • 9.3087 — rounded an intermediate value before the final step.

Reference: FE Handbook — Langmuir Isotherm

Example 2
Langmuir Isotherm — solve for equilibrium concentration — Langmuir Isotherm (2)

Langmuir isotherm linearized form used to determine adsorption constants Given maximum adsorption capacity (a) = 163.0 mg/g; Langmuir equilibrium constant (b) = 1.9480 L/mg; adsorption capacity (x/m) (x_m) = 132.2 mg/g, determine the equilibrium concentration (C_e) in mg/L.

Given

  • maximumadsorptioncapacity(a)=163.0mg/gmaximum adsorption capacity (a) = 163.0 mg/g
  • Langmuirequilibriumconstant(b)=1.9480L/mgLangmuir equilibrium constant (b) = 1.9480 L/mg
  • adsorptioncapacity(x/m)(xm)=132.2mg/gadsorption capacity (x/m) (x_m) = 132.2 mg/g

Find

equilibrium concentration (C_e), in mg/L

Start with the thinking

  • The governing relation printed in this handbook section is Langmuir Isotherm.
  • Everything except C_e is given, so isolate C_e symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The Langmuir isotherm describes monolayer adsorption capacity and can be expressed in a linearized form for parameter fitting.
1/Ce1/(x/m)Langmuir isotherm linearized form

Figure 2 — schematic for Langmuir Isotherm — solve for equilibrium concentration — Langmuir Isotherm (2)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}
  2. Step 2 — Rearrange symbolically for C_e:

    Ce=x/mab−(x/m)bC_{e} = \dfrac{x/m}{ab-(x/m)b}
  3. Step 3 — List the givens: maximum adsorption capacity (a) = 163.0 mg/g, Langmuir equilibrium constant (b) = 1.9480 L/mg, adsorption capacity (x/m) (x_m) = 132.2 mg/g.

  4. Step 4 — Substitute the given values:

    Ce=x/ma1.9480−(x/m)1.9480C_{e} = \dfrac{x/m}{a1.9480-(x/m)1.9480}
  5. Step 5 — Evaluate:

    Ce=2.2034 mg/LC_{e} = 2.2034\ \text{mg/L}
  6. Step 6 — Check: returning C_e = 2.2034 mg/L to

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}

    reproduces the given quantities, and both sides carry the same units.

Answer:
Ce=2.2034 mg/LC_{e} = 2.2034\ \text{mg/L}

Why the other options are there

  • 4.4068 — kept a factor of two that cancels in the correct rearrangement.
  • 1.1017 — dropped that same factor in the other direction.
  • 2.4237 — rounded an intermediate value before the final step.

Reference: FE Handbook — Langmuir Isotherm

Example 3
Langmuir Isotherm — solve for maximum adsorption capacity — Langmuir Isotherm (3)

Langmuir isotherm adsorption capacity for water treatment design Given Langmuir equilibrium constant (b) = 1.2160 L/mg; equilibrium concentration (C_e) = 4.9000 mg/L; adsorption capacity (x/m) (x_m) = 169.5 mg/g, determine the maximum adsorption capacity (a) in mg/g.

Given

  • Langmuirequilibriumconstant(b)=1.2160L/mgLangmuir equilibrium constant (b) = 1.2160 L/mg
  • equilibriumconcentration(Ce)=4.9000mg/Lequilibrium concentration (C_e) = 4.9000 mg/L
  • adsorptioncapacity(x/m)(xm)=169.5mg/gadsorption capacity (x/m) (x_m) = 169.5 mg/g

Find

maximum adsorption capacity (a), in mg/g

Start with the thinking

  • The governing relation printed in this handbook section is Langmuir Isotherm.
  • Everything except a is given, so isolate a symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The Langmuir isotherm describes monolayer adsorption capacity and can be expressed in a linearized form for parameter fitting.
1/Ce1/(x/m)Langmuir isotherm linearized form

Figure 3 — schematic for Langmuir Isotherm — solve for maximum adsorption capacity — Langmuir Isotherm (3)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}
  2. Step 2 — Rearrange symbolically for a:

    a=(x/m)(1+bCe)bCea = \dfrac{(x/m)(1+bC_e)}{bC_e}
  3. Step 3 — List the givens: Langmuir equilibrium constant (b) = 1.2160 L/mg, equilibrium concentration (C_e) = 4.9000 mg/L, adsorption capacity (x/m) (x_m) = 169.5 mg/g.

  4. Step 4 — Substitute the given values:

    a=(x/m)(1+bCe)bCea = \dfrac{(x/m)(1+bC_e)}{bC_e}
  5. Step 5 — Evaluate:

    a=197.9 mg/ga = 197.9\ \text{mg/g}
  6. Step 6 — Check: returning a = 197.9 mg/g to

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}

    reproduces the given quantities, and both sides carry the same units.

Answer:
a=197.9 mg/ga = 197.9\ \text{mg/g}

Why the other options are there

  • 395.9 — kept a factor of two that cancels in the correct rearrangement.
  • 98.9736 — dropped that same factor in the other direction.
  • 217.7 — rounded an intermediate value before the final step.

Reference: FE Handbook — Langmuir Isotherm

Example 4
Langmuir Isotherm — solve for adsorption capacity (x/m) (case 2) — Langmuir Isotherm (4)

Langmuir isotherm monolayer adsorption model for carbon treatment Given maximum adsorption capacity (a) = 104.0 mg/g; Langmuir equilibrium constant (b) = 1.7520 L/mg; equilibrium concentration (C_e) = 22.9000 mg/L, determine the adsorption capacity (x/m) (x_m) in mg/g.

Given

  • maximumadsorptioncapacity(a)=104.0mg/gmaximum adsorption capacity (a) = 104.0 mg/g
  • Langmuirequilibriumconstant(b)=1.7520L/mgLangmuir equilibrium constant (b) = 1.7520 L/mg
  • equilibriumconcentration(Ce)=22.9000mg/Lequilibrium concentration (C_e) = 22.9000 mg/L

Find

adsorption capacity (x/m) (x_m), in mg/g

Start with the thinking

  • The governing relation printed in this handbook section is Langmuir Isotherm.
  • Everything except x_m is given, so isolate x_m symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The Langmuir isotherm describes monolayer adsorption capacity and can be expressed in a linearized form for parameter fitting.
1/Ce1/(x/m)Langmuir isotherm linearized form

Figure 4 — schematic for Langmuir Isotherm — solve for adsorption capacity (x/m) (case 2) — Langmuir Isotherm (4)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}
  2. Step 2 — Rearrange symbolically for x_m:

    xm=abCe1+bCex_{m} = \dfrac{abC_e}{1+bC_e}
  3. Step 3 — List the givens: maximum adsorption capacity (a) = 104.0 mg/g, Langmuir equilibrium constant (b) = 1.7520 L/mg, equilibrium concentration (C_e) = 22.9000 mg/L.

  4. Step 4 — Substitute the given values:

    xm=abCe1+bCex_{m} = \dfrac{abC_e}{1+bC_e}
  5. Step 5 — Evaluate:

    xm=101.5 mg/gx_{m} = 101.5\ \text{mg/g}
  6. Step 6 — Check: returning x_m = 101.5 mg/g to

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}

    reproduces the given quantities, and both sides carry the same units.

Answer:
xm=101.5 mg/gx_{m} = 101.5\ \text{mg/g}

Why the other options are there

  • 202.9 — kept a factor of two that cancels in the correct rearrangement.
  • 50.7354 — dropped that same factor in the other direction.
  • 111.6 — rounded an intermediate value before the final step.

Reference: FE Handbook — Langmuir Isotherm

Example 5
Langmuir Isotherm — solve for equilibrium concentration (case 2) — Langmuir Isotherm (5)

Langmuir isotherm linearized form used to determine adsorption constants Given maximum adsorption capacity (a) = 159.0 mg/g; Langmuir equilibrium constant (b) = 0.5890 L/mg; adsorption capacity (x/m) (x_m) = 86.7000 mg/g, determine the equilibrium concentration (C_e) in mg/L.

Given

  • maximumadsorptioncapacity(a)=159.0mg/gmaximum adsorption capacity (a) = 159.0 mg/g
  • Langmuirequilibriumconstant(b)=0.5890L/mgLangmuir equilibrium constant (b) = 0.5890 L/mg
  • adsorptioncapacity(x/m)(xm)=86.7000mg/gadsorption capacity (x/m) (x_m) = 86.7000 mg/g

Find

equilibrium concentration (C_e), in mg/L

Start with the thinking

  • The governing relation printed in this handbook section is Langmuir Isotherm.
  • Everything except C_e is given, so isolate C_e symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The Langmuir isotherm describes monolayer adsorption capacity and can be expressed in a linearized form for parameter fitting.
1/Ce1/(x/m)Langmuir isotherm linearized form

Figure 5 — schematic for Langmuir Isotherm — solve for equilibrium concentration (case 2) — Langmuir Isotherm (5)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}
  2. Step 2 — Rearrange symbolically for C_e:

    Ce=x/mab−(x/m)bC_{e} = \dfrac{x/m}{ab-(x/m)b}
  3. Step 3 — List the givens: maximum adsorption capacity (a) = 159.0 mg/g, Langmuir equilibrium constant (b) = 0.5890 L/mg, adsorption capacity (x/m) (x_m) = 86.7000 mg/g.

  4. Step 4 — Substitute the given values:

    Ce=x/ma0.5890−(x/m)0.5890C_{e} = \dfrac{x/m}{a0.5890-(x/m)0.5890}
  5. Step 5 — Evaluate:

    Ce=2.0359 mg/LC_{e} = 2.0359\ \text{mg/L}
  6. Step 6 — Check: returning C_e = 2.0359 mg/L to

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}

    reproduces the given quantities, and both sides carry the same units.

Answer:
Ce=2.0359 mg/LC_{e} = 2.0359\ \text{mg/L}

Why the other options are there

  • 4.0719 — kept a factor of two that cancels in the correct rearrangement.
  • 1.0180 — dropped that same factor in the other direction.
  • 2.2395 — rounded an intermediate value before the final step.

Reference: FE Handbook — Langmuir Isotherm

Example 6
Langmuir Isotherm — solve for maximum adsorption capacity (case 2) — Langmuir Isotherm (6)

Langmuir isotherm adsorption capacity for water treatment design Given Langmuir equilibrium constant (b) = 1.9880 L/mg; equilibrium concentration (C_e) = 42.7000 mg/L; adsorption capacity (x/m) (x_m) = 65.0000 mg/g, determine the maximum adsorption capacity (a) in mg/g.

Given

  • Langmuirequilibriumconstant(b)=1.9880L/mgLangmuir equilibrium constant (b) = 1.9880 L/mg
  • equilibriumconcentration(Ce)=42.7000mg/Lequilibrium concentration (C_e) = 42.7000 mg/L
  • adsorptioncapacity(x/m)(xm)=65.0000mg/gadsorption capacity (x/m) (x_m) = 65.0000 mg/g

Find

maximum adsorption capacity (a), in mg/g

Start with the thinking

  • The governing relation printed in this handbook section is Langmuir Isotherm.
  • Everything except a is given, so isolate a symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The Langmuir isotherm describes monolayer adsorption capacity and can be expressed in a linearized form for parameter fitting.
1/Ce1/(x/m)Langmuir isotherm linearized form

Figure 6 — schematic for Langmuir Isotherm — solve for maximum adsorption capacity (case 2) — Langmuir Isotherm (6)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}
  2. Step 2 — Rearrange symbolically for a:

    a=(x/m)(1+bCe)bCea = \dfrac{(x/m)(1+bC_e)}{bC_e}
  3. Step 3 — List the givens: Langmuir equilibrium constant (b) = 1.9880 L/mg, equilibrium concentration (C_e) = 42.7000 mg/L, adsorption capacity (x/m) (x_m) = 65.0000 mg/g.

  4. Step 4 — Substitute the given values:

    a=(x/m)(1+bCe)bCea = \dfrac{(x/m)(1+bC_e)}{bC_e}
  5. Step 5 — Evaluate:

    a=65.7657 mg/ga = 65.7657\ \text{mg/g}
  6. Step 6 — Check: returning a = 65.7657 mg/g to

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}

    reproduces the given quantities, and both sides carry the same units.

Answer:
a=65.7657 mg/ga = 65.7657\ \text{mg/g}

Why the other options are there

  • 131.5 — kept a factor of two that cancels in the correct rearrangement.
  • 32.8829 — dropped that same factor in the other direction.
  • 72.3423 — rounded an intermediate value before the final step.

Reference: FE Handbook — Langmuir Isotherm

Example 7
Langmuir Isotherm — solve for adsorption capacity (x/m) (case 3) — Langmuir Isotherm (7)

Langmuir isotherm monolayer adsorption model for carbon treatment Given maximum adsorption capacity (a) = 108.0 mg/g; Langmuir equilibrium constant (b) = 0.8500 L/mg; equilibrium concentration (C_e) = 2.1000 mg/L, determine the adsorption capacity (x/m) (x_m) in mg/g.

Given

  • maximumadsorptioncapacity(a)=108.0mg/gmaximum adsorption capacity (a) = 108.0 mg/g
  • Langmuirequilibriumconstant(b)=0.8500L/mgLangmuir equilibrium constant (b) = 0.8500 L/mg
  • equilibriumconcentration(Ce)=2.1000mg/Lequilibrium concentration (C_e) = 2.1000 mg/L

Find

adsorption capacity (x/m) (x_m), in mg/g

Start with the thinking

  • The governing relation printed in this handbook section is Langmuir Isotherm.
  • Everything except x_m is given, so isolate x_m symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The Langmuir isotherm describes monolayer adsorption capacity and can be expressed in a linearized form for parameter fitting.
1/Ce1/(x/m)Langmuir isotherm linearized form

Figure 7 — schematic for Langmuir Isotherm — solve for adsorption capacity (x/m) (case 3) — Langmuir Isotherm (7)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}
  2. Step 2 — Rearrange symbolically for x_m:

    xm=abCe1+bCex_{m} = \dfrac{abC_e}{1+bC_e}
  3. Step 3 — List the givens: maximum adsorption capacity (a) = 108.0 mg/g, Langmuir equilibrium constant (b) = 0.8500 L/mg, equilibrium concentration (C_e) = 2.1000 mg/L.

  4. Step 4 — Substitute the given values:

    xm=abCe1+bCex_{m} = \dfrac{abC_e}{1+bC_e}
  5. Step 5 — Evaluate:

    xm=69.2208 mg/gx_{m} = 69.2208\ \text{mg/g}
  6. Step 6 — Check: returning x_m = 69.2208 mg/g to

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}

    reproduces the given quantities, and both sides carry the same units.

Answer:
xm=69.2208 mg/gx_{m} = 69.2208\ \text{mg/g}

Why the other options are there

  • 138.4 — kept a factor of two that cancels in the correct rearrangement.
  • 34.6104 — dropped that same factor in the other direction.
  • 76.1429 — rounded an intermediate value before the final step.

Reference: FE Handbook — Langmuir Isotherm

Example 8
Langmuir Isotherm — solve for equilibrium concentration (case 3) — Langmuir Isotherm (8)

Langmuir isotherm linearized form used to determine adsorption constants Given maximum adsorption capacity (a) = 133.0 mg/g; Langmuir equilibrium constant (b) = 0.1920 L/mg; adsorption capacity (x/m) (x_m) = 102.7 mg/g, determine the equilibrium concentration (C_e) in mg/L.

Given

  • maximumadsorptioncapacity(a)=133.0mg/gmaximum adsorption capacity (a) = 133.0 mg/g
  • Langmuirequilibriumconstant(b)=0.1920L/mgLangmuir equilibrium constant (b) = 0.1920 L/mg
  • adsorptioncapacity(x/m)(xm)=102.7mg/gadsorption capacity (x/m) (x_m) = 102.7 mg/g

Find

equilibrium concentration (C_e), in mg/L

Start with the thinking

  • The governing relation printed in this handbook section is Langmuir Isotherm.
  • Everything except C_e is given, so isolate C_e symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The Langmuir isotherm describes monolayer adsorption capacity and can be expressed in a linearized form for parameter fitting.
1/Ce1/(x/m)Langmuir isotherm linearized form

Figure 8 — schematic for Langmuir Isotherm — solve for equilibrium concentration (case 3) — Langmuir Isotherm (8)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}
  2. Step 2 — Rearrange symbolically for C_e:

    Ce=x/mab−(x/m)bC_{e} = \dfrac{x/m}{ab-(x/m)b}
  3. Step 3 — List the givens: maximum adsorption capacity (a) = 133.0 mg/g, Langmuir equilibrium constant (b) = 0.1920 L/mg, adsorption capacity (x/m) (x_m) = 102.7 mg/g.

  4. Step 4 — Substitute the given values:

    Ce=x/ma0.1920−(x/m)0.1920C_{e} = \dfrac{x/m}{a0.1920-(x/m)0.1920}
  5. Step 5 — Evaluate:

    Ce=17.6533 mg/LC_{e} = 17.6533\ \text{mg/L}
  6. Step 6 — Check: returning C_e = 17.6533 mg/L to

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}

    reproduces the given quantities, and both sides carry the same units.

Answer:
Ce=17.6533 mg/LC_{e} = 17.6533\ \text{mg/L}

Why the other options are there

  • 35.3067 — kept a factor of two that cancels in the correct rearrangement.
  • 8.8267 — dropped that same factor in the other direction.
  • 19.4187 — rounded an intermediate value before the final step.

Reference: FE Handbook — Langmuir Isotherm

Example 9
Langmuir Isotherm — solve for maximum adsorption capacity (case 3) — Langmuir Isotherm (9)

Langmuir isotherm adsorption capacity for water treatment design Given Langmuir equilibrium constant (b) = 0.7660 L/mg; equilibrium concentration (C_e) = 11.4000 mg/L; adsorption capacity (x/m) (x_m) = 27.8000 mg/g, determine the maximum adsorption capacity (a) in mg/g.

Given

  • Langmuirequilibriumconstant(b)=0.7660L/mgLangmuir equilibrium constant (b) = 0.7660 L/mg
  • equilibriumconcentration(Ce)=11.4000mg/Lequilibrium concentration (C_e) = 11.4000 mg/L
  • adsorptioncapacity(x/m)(xm)=27.8000mg/gadsorption capacity (x/m) (x_m) = 27.8000 mg/g

Find

maximum adsorption capacity (a), in mg/g

Start with the thinking

  • The governing relation printed in this handbook section is Langmuir Isotherm.
  • Everything except a is given, so isolate a symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The Langmuir isotherm describes monolayer adsorption capacity and can be expressed in a linearized form for parameter fitting.
1/Ce1/(x/m)Langmuir isotherm linearized form

Figure 9 — schematic for Langmuir Isotherm — solve for maximum adsorption capacity (case 3) — Langmuir Isotherm (9)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}
  2. Step 2 — Rearrange symbolically for a:

    a=(x/m)(1+bCe)bCea = \dfrac{(x/m)(1+bC_e)}{bC_e}
  3. Step 3 — List the givens: Langmuir equilibrium constant (b) = 0.7660 L/mg, equilibrium concentration (C_e) = 11.4000 mg/L, adsorption capacity (x/m) (x_m) = 27.8000 mg/g.

  4. Step 4 — Substitute the given values:

    a=(x/m)(1+bCe)bCea = \dfrac{(x/m)(1+bC_e)}{bC_e}
  5. Step 5 — Evaluate:

    a=30.9835 mg/ga = 30.9835\ \text{mg/g}
  6. Step 6 — Check: returning a = 30.9835 mg/g to

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}

    reproduces the given quantities, and both sides carry the same units.

Answer:
a=30.9835 mg/ga = 30.9835\ \text{mg/g}

Why the other options are there

  • 61.9671 — kept a factor of two that cancels in the correct rearrangement.
  • 15.4918 — dropped that same factor in the other direction.
  • 34.0819 — rounded an intermediate value before the final step.

Reference: FE Handbook — Langmuir Isotherm

Example 10
Langmuir Isotherm — solve for adsorption capacity (x/m) (case 4) — Langmuir Isotherm (10)

Langmuir isotherm monolayer adsorption model for carbon treatment Given maximum adsorption capacity (a) = 100.0 mg/g; Langmuir equilibrium constant (b) = 1.7410 L/mg; equilibrium concentration (C_e) = 0.9000 mg/L, determine the adsorption capacity (x/m) (x_m) in mg/g.

Given

  • maximumadsorptioncapacity(a)=100.0mg/gmaximum adsorption capacity (a) = 100.0 mg/g
  • Langmuirequilibriumconstant(b)=1.7410L/mgLangmuir equilibrium constant (b) = 1.7410 L/mg
  • equilibriumconcentration(Ce)=0.9000mg/Lequilibrium concentration (C_e) = 0.9000 mg/L

Find

adsorption capacity (x/m) (x_m), in mg/g

Start with the thinking

  • The governing relation printed in this handbook section is Langmuir Isotherm.
  • Everything except x_m is given, so isolate x_m symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The Langmuir isotherm describes monolayer adsorption capacity and can be expressed in a linearized form for parameter fitting.
1/Ce1/(x/m)Langmuir isotherm linearized form

Figure 10 — schematic for Langmuir Isotherm — solve for adsorption capacity (x/m) (case 4) — Langmuir Isotherm (10)

Step-by-step solution

  1. Step 1 — State the governing relation:

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}
  2. Step 2 — Rearrange symbolically for x_m:

    xm=abCe1+bCex_{m} = \dfrac{abC_e}{1+bC_e}
  3. Step 3 — List the givens: maximum adsorption capacity (a) = 100.0 mg/g, Langmuir equilibrium constant (b) = 1.7410 L/mg, equilibrium concentration (C_e) = 0.9000 mg/L.

  4. Step 4 — Substitute the given values:

    xm=abCe1+bCex_{m} = \dfrac{abC_e}{1+bC_e}
  5. Step 5 — Evaluate:

    xm=61.0425 mg/gx_{m} = 61.0425\ \text{mg/g}
  6. Step 6 — Check: returning x_m = 61.0425 mg/g to

    xm=abCe1+bCe\dfrac{x}{m} = \dfrac{a b C_e}{1 + b C_e}

    reproduces the given quantities, and both sides carry the same units.

Answer:
xm=61.0425 mg/gx_{m} = 61.0425\ \text{mg/g}

Why the other options are there

  • 122.1 — kept a factor of two that cancels in the correct rearrangement.
  • 30.5213 — dropped that same factor in the other direction.
  • 67.1468 — rounded an intermediate value before the final step.

Reference: FE Handbook — Langmuir Isotherm

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