1. For the reaction 4A + 3B ----> 5C + 3F you are given 7.0 mol of A and 5.0 mol

Question

1. For the reaction 4A + 3B ----> 5C + 3F you are given 7.0 mol of A and 5.0 mol of B. The reaction does not go to completion. It stops when you have only made 2.0 moles of C. How many moles of B are left unused?

2. Oxalic acid, H2C2O4 (MW 90.0 g/mol), is found in spinach. Spinach juice is often tested to make sure the oxalic acid concentration isn't very high because of the toxicity of the acid. A 50.0 g sample of spinach juice required 23.7 mL of 0.127 M permanganante solution to reach the end point. What is the % oxalic acid in the juice?

Explanation / Answer

1) 4A + 3B ----> 5C + 3F

we have 7.0 mol of A and 5.0 mol of B.

moles of B used = 3 * 2.0 / 5 = 1.2 mole.

so moles of B are left unused = (5.0 - 1.2) = 3.8 mole B

2) reaction is

2 KMnO4 + 5 C2O4H2 + 3 H2SO4 ........................... > 2 MnSO4 + 10 CO2 + 8 H2O + K2SO4

23.7 mL of 0.127 M permanganante = 0.0237 * 0.127 = 3.01 * 10^-3 mole.

so mass of oxalic acid in spinach = 5 *  3.01 * 10^-3 / 2 = 7.52 * 10^-3 mole = 7.52 * 10^-3 * 90.0 = 0.679 gm.

% oxalic acid in the juice = 0.679 * 100 / 50.0 = 1.36 %

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