2. How many grams of solid lithium must be added to liquid water in order to obtain 15.0 L of
hydrogen gas at STP? ( 9.29g)
2 Li + 2H20 → 2LiOH + H2

9.4g

Explanation:

Given parameters:

Volume of hydrogen gas at STP  = 15L

Unknown:

Mass of solid lithium = ?

Solution:

The reaction equation is shown below;

                      2Li    +     2H₂O   →     2LiOH    +    H₂

To solve this problem, lets work from the known to the unknown. The known here is hydrogen gas;

          At STP;

             number of moles  =   =   = 0.67mole

From the balanced reaction equation;

      1 mole of hydrogen gas is produced from 2 moles of Li;

     0.67 moles of hydrogen will be produced from 0.67 x 2  = 1.34moles

Mass of solid lithium  = number of moles x molar mass

                                      = 1.34  x 7

                                      =9.4g