If you react 0.025 moles of 2-methylbromobenzene, 0.7 grams of Mg, and 10 grams of dry ice, what is the theoretical yield of 2-methylbenzoic acid in grams?

Explanation:

1. Write the overall equation.

The Grignard reaction has several steps:

(a) CH₃C₆H₄Br + Mg ⟶ CH₃C₆H₄MgBr

(b) CH₃C₆H₄MgBr + CO₂ ⟶ CH₃C₆H₄COOMgBr

(c) CH₃C₆H₄COOMgBr  + H⁺ ⟶ CH₃C₆H₄COOH + Mg²⁺ + Br⁻    

   CH₃C₆H₄Br + Mg + CO₂ + H⁺ ⟶ CH₃C₆H₄COOH + Mg²⁺ + Br⁻

Now we can gather all the information in one place.

Mᵣ:              171.03         24.31   44.01                        136.15

              CH₃C₆H₄Br +   Mg  +   CO₂ + H⁺ ⟶ CH₃C₆H₄COOH + Mg²⁺ + Br⁻

n/mol:        0.025

m/g:                                 0.7        10

Let's call CH₃C₆H₄Br "R" (reactant) and CH₃C₆H₄COOH "P" (product).

They have given us the amounts of three reactants and asked us to calculate the amount of product. This is a limiting reactant problem.

1. Calculate the moles of each reactant

(a) CH₃C₆H₄Br

n = 0.025 mol (given)

(b) Mg

(c) CO₂

2. Calculate the moles of CH₃C₆H₄COOH you can obtain from each reactant

The molar ratios are all 1:1, so

(a) 0.025 mol R    ⟶ 0.025 mol P

(b) 0.028 mol Mg ⟶ 0.029 mol P

(c) 0.23 mol CO₂  ⟶ 0.23    mol P

R is the limiting reactant, because it gives the fewest moles of P.

3. Calculate the theoretical yield.

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