How many grams of calcium must react with sulfuric acid , H2SO4 , in order to produce 6.5 liters of hydrogen gas ?

Question

Ca(s)  + H2SO4(aq) --> CaSO4(aq) + H2(g)

Answer 1

Since the standard molar volume is 22.4 L   ,   6.5 / 22.4  =  0.29 moles of H2 are required.

The chemical equation says the one mole of H2 is produced from each mole of Ca.   Thus 0.29 moles of Ca must react.

The molar mass of Ca is approximately 40.0 g / mole.   Therefore 0.29 x 40 = 11.6 g of Ca are required. 

Answer 2

Ca(s) + H2SO4(aq) --> CaSO4(aq) + H2(g)

Since the question is about a gas, the answer will depend on the temperature and pressure of the system.  In view of the fact that no information is provided regarding these two parameters, one must assume the system is at STP (Standard Temperature and Pressure), which is 0ºC and 1 atm.  At STP the volume of any ideal occupied by 1 mole = 22.4 liters.

 

6.5 L H2(g) x 1 mole/22.4 L = 0.290 moles H2

0.290 moles H2 x 1 mole Ca/mole H2 = 0.290 moles Ca 0.290 moles Ca x 40 g/mole = 11.6 g Ca = 12 g Ca (to 2 sig. figs).