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Question

1.3g of zinc metal is dropped into 15g of hydrochloric acid (HCl) to produce a gas at STP. After the reaction, some acid remained. Find the mass of the remaining acid. 

Answer 1

This is essentially a limiting reactant problem. First you must write a correctly balanced equation for the reaction taking place.

Zn(s) + 2HCl(aq) ==> ZnCl2(aq) + H2(g) ... balanced equation

To find the limiting reactant, simply divide the mols of each reactant by the corresponding coefficient in the balanced equation and which ever result is less represents the limiting reactant.

For Zn: 1.3 g Zn x 1 mol Zn/65.39 g = 0.0199 mols Zn (÷1->0.0199)

For HCl: 15 g HCl x 1 mol HCl/36.5 g = 0.422 mols HCl (÷2->0.205)

Zn is limiting since 0.0199 is less than 0.205)

To find the mass of HCl remaining, we'll find moles of HCl used up, then moles HCl remaining, and then mass of HCl remaining.

moles HCl used up = 0.0199 mols Zn x 2 mols HCl / 1 mol Zn = 0.0398 mols HCl used up

moles HCl remaining = 0.422 mols HCl - 0.0398 mols HCl = 0.382 mols HCl remaining

mass HCl remaining = 0.382 mols HCl x 36.5 g/mol = 13.9 g HCl remaining