# How do you calculate mass percent?

Submitted by danie_m2 on Fri, 11/30/2012 - 12:28

How do I calculate the mass percent of a compound after the compound has been titrated using another chemical solution?

Example:

1. A sample of limestone weighing 1.005 g is dissolved in 75.00 mL of 0.2500 M hydrochloric acid. The following reaction occurs:

CaCO3(s)   + 2H+(aq)  àCa2+(aq)   +  CO2(g)  +   H2O

It is found that 19.26 mL of 0.150M NaOH is required to titrate the excess HCl left after the reaction with the limestone. What is the mass percent of CaCO3 in the limestone

I know that mass percent is calculated by the total mass of an element over the mass of the compound and then multiplied by 100%, but if they're asking for the total mass percent of the compound wouldn't that just be 100%?

Find moles NaOH used from volume in L x molarity. Each mole NaOH reacts with 1 mole HCl (HCl + NaOH ---> NaCl + H2O). So you have the number of moles of HCl that were left over after reacting with CaCO3. Find moles HCl added at the start from volume in L x molarity. Subtract moles HCl left over from moles HCl at the start to get the number of moles of HCl (I'll call this w moles) that reacted with the CaCO3. From this balanced equation 2 moles HCl reacts with 1 mole CaCO3, so w moles HCl would react with 0,5w moles CaCO3. Next find the mass of 0.5w moles CaCO3 - this is the actual mass of CaCO3 present in the 1.005g of impure CaCO3 you started with. (Molar mass of CaCO3 is approx 40 + 12 + 16x3 = 100g. So you have 0.5w x 100 g of CaCO3

Mass percent = (actual mass of CaCO3/starting mass of CaCO3) x 100