Chemistry: Stoichometry

Question

Lithium metal decomposes in water and releases hydrogen gas. If 275 mL of hydrogen gas are produced at STP, what is the mass of lithium that reacted.

 

Li + H2O -> LiOH + H2

Answer 1

Li + H₂O -> LiOH + H₂

At STP 1 mole of an ideal gas = 22.4 liters.  First convert 275 ml of H₂ gas to moles using this conversion.

275 ml gas x 1 L/1000 ml x 1 mole/22.4 L = 0.01228 moles H₂ gas 

From the balanced equation, it is seen that 1 mole Li makes 1 mole H₂ gas.  Now, find moles of Li.

moles Li = 0.01228 moles H₂ x 1 mole Li/mole H₂ = 0.01228 moles Li

Finally convert moles Li to grams Li.

0.01228 moles Li x 6.94 g/mole = 0.0852 g Li