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How to Calculate Mass of Theoretical Yield?

Published in Stoichiometry 3 mins read

The mass of the theoretical yield is calculated by multiplying the moles of the theoretical yield (product) by its molar mass.

Here's a breakdown of the steps involved in determining the mass of the theoretical yield:

1. Write and Balance the Chemical Equation:

  • Start by writing the balanced chemical equation for the reaction. This ensures you have the correct stoichiometry (mole ratios) between reactants and products.

2. Determine the Limiting Reactant:

  • The limiting reactant is the reactant that is completely consumed first, thus limiting the amount of product that can be formed.
  • To identify the limiting reactant:
    • Convert the mass of each reactant to moles (using their respective molar masses).
    • Divide the moles of each reactant by its stoichiometric coefficient from the balanced equation.
    • The reactant with the smallest result is the limiting reactant.

3. Calculate the Theoretical Yield in Moles:

  • Use the stoichiometry of the balanced equation to determine the mole ratio between the limiting reactant and the product you are interested in.
  • Multiply the moles of the limiting reactant by this mole ratio to find the theoretical yield of the product in moles.

4. Calculate the Theoretical Yield in Grams (Mass):

  • Multiply the theoretical yield in moles (calculated in step 3) by the molar mass of the product. This gives you the theoretical yield in grams.

Formula:

Theoretical Yield (grams) = Theoretical Yield (moles) × Molar Mass of Product (g/mol)

Example:

Consider the reaction: 2H2(g) + O2(g) → 2H2O(g)

Suppose you react 4 grams of H2 with 32 grams of O2.

  1. Moles of H2: 4 g / 2.016 g/mol = 1.98 moles
  2. Moles of O2: 32 g / 32.00 g/mol = 1 mole

Dividing by the coefficients in the balanced equation:

  • H2: 1.98 moles / 2 = 0.99
  • O2: 1 mole / 1 = 1

H2 is the limiting reactant.

Theoretical Yield of H2O (moles):

  • From the balanced equation, 2 moles of H2 produce 2 moles of H2O. The mole ratio is 1:1.
  • Therefore, 1.98 moles of H2 will produce 1.98 moles of H2O.

Theoretical Yield of H2O (grams):

  • Molar mass of H2O = 18.015 g/mol
  • Theoretical yield (grams) = 1.98 moles × 18.015 g/mol = 35.67 g

Therefore, the theoretical yield of water in this example is 35.67 grams.

In summary, calculating the mass of the theoretical yield involves identifying the limiting reactant, calculating the moles of the desired product it can produce based on stoichiometry, and then converting those moles into grams using the product's molar mass.

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