chemistry question

Question

Some underwater welding is done via the thermite reaction, in which rust (Fe2O3) reacts with aluminum (Al) to produce iron (Fe) and aluminum oxide (Al2O3). In one such reaction, 258 g of aluminum and excess rust actually produced 464 g of iron.

Fe2O3 + Al ------------------------> Al2O3 + Fe

a. What is the theoretical yield of Fe?

b. What is the % yield of iron?

Answer 1

Step 1: Balance

When given a chemical reaction, your first thought should always be to balance the reaction. For this double replacement reaction, simply add coefficients of 2 in front of aluminum and iron:

Fe₂O₃ + 2 Al --> Al₂O₃ + 2 Fe

Step 2: Use stoichiometry to predict the mass of iron

Now, you'll need to use the starting mass of aluminum (258 g) to predict the mass of iron that would be produced, in a theoretical (perfect) situation. (Because rust, Fe₂O₃, is listed as an excess reactant, you can assume it will not impact your final result).

First, convert the mass of aluminum from grams to moles by dividing by its molar mass:

258 g Al ÷ 26.982 g/mol = 9.56 mol Al

Then, use the mole ratios/coefficients from the balanced equation to convert from moles of Al to moles of Fe:

9.56 mol Al x (2 mol Fe / 2 mol Al) = 9.56 mol Fe

This is an easy conversion because for every 2 moles of aluminum, you will produce 2 moles of iron. It is a 1:1 conversion!

Finally, convert the moles of iron to grams by multiplying by its molar mass. (Whenever converting from moles to mass, remember to "mole-tiply").

9.56 mol Fe x 55.845 g/mol = 533.946 g Fe

a. The theoretical yield is the mass of product expected or 533 grams of iron. Note: this value has been rounded to match the sig figs in the initial value given in the problem. Because the mass of aluminum (258) had 3 significant digits, your answer should include the same number of digits.

b. Percent yield

The percent yield shows how close the experimental yield is to the ideal/perfect yield. Use the following equation:

% yield = (Experimental Yield/ Theoretical Yield) × 100%

% yield = (464 g/533 g) × 100% = 87.0544 %

Again, you'll want to round your final answer to just 3 sig figs = 87.1%

Hope that helps!