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