Question

ithium metal reacts with water to give lithium hydroxide and hydrogen gas. if 75.5 mL of hydrogen gas is produce at STP, what is the mass of lithium metal that reacted?

Answer 1

The reaction would written as:

2Li + 2H2O = 2LiOH + H2

We are given the amount in volume of the hydrogen gas that is produced. Assuming that this gas is an ideal gas, we use the relation that in every 1 mol of gas, 22.4 L is being occupied.

75.5 mL ( 1 L / 1000 mL ) ( 1 mol / 22.4 L ) = 0.0034 mol H2 produced

0.0034 mol H2 ( 2 mol Li / 1 mol H2 ) ( 6.941 g / mol ) = 0.0472 g Li is needed

Answer 2

The reaction equation may be written as:
2Li + 2H₂O → 2LiOH + H₂

A single mole of gas occupies 22,400 ml of gas at STP. Therefore, the moles of hydrogen gas are
75.5 / 22,400 = 0.0034 mol
According to the equation, the moles of lithium are twice that of hydrogen gas; thus,
0.0034 x 2 = 0.0068 mol

The mass of lithium is given by:
mass = moles x atomic mass
mass = 0.0068 x 6.9
mass = 0.047 grams