Question

how to solve a percent yield problem using a mass-to-mass stoichiometry.

Answer 1

Answer:

There are four steps involved in solving these problems:

Make sure you are working with a properly balanced chemical equation.

Convert grams of the substance given in the problem to moles.

Construct two ratios - one from the problem and one from the chemical equation and set them equal. The ratio from the problem will have an unknown, 'x.' Solve for "x."

Convert moles of the substance just solved for into grams.

Step-by-step explanation:

Example #1: How many grams of hydrogen gas are needed to react completely with 54.0 g of oxygen gas, given the following unbalanced chemical reaction:

H2 + O2 ---> H2O

Solution:

1) Balance the chemical equation:

2H2 + O2 ---> 2H2O

2) Convert grams of the substance given:

54.0 g / 32.0 g/mol = 1.6875 mol of O2

Note the use of 32.0 and not 16.0. The chemical substance is O2. Students have been known to sometimes forget to write the subscript of 2 on a diatomic element (H2, N2, O2, F2, Cl2, Br2, I2)

3) Construct two molar ratios and set them equal to each other:

First molar ratio is from the coefficients of the balanced chemical equation. The two substances are:

H2–––O2

and the numerical ratio is this:

2–––1

The second ratio is found within the problem statement. The H2 is our unknown because the problem says "how many grams of hydrogen" and the O2 mole amount is the other value. Like this:

x–––––1.6875

I left the mol unit off for convenience. Note also that I did not round off. I'll do that at the end.

We need to set the two ratios equal to each other and solve:

2 x–––=–––––1 1.6875

x = 3.375 mol of H2 required

4) Convert the calculated moles from step #3 into grams:

(3.375 mol) (2.016 g/mol) = 6.80 g (to three sig figs)

Note: if you did not balance the equation, you'd wind up using an incorrect 1:1 molar ratio rather than the correct 2:1 ratio.