Your theoretical yield will be
Once again, start with the balanced chemical equation
In other words, regardless of how many hydrogen moles react, you'll always have 2/3 less moles of ammonia produced.
Determine the number of moles of hydrogen gas by using its molar mass
For a 100% yield, all the moles of hydrogen gas must react to produce ammonia. This means that the moles of ammonia produced will be
Use ammonia's molar mass to see how many grams would be produced in a 100%-yield reaction
However, your reaction produces less ammonia (26.7 g) than what was calculated for a 100% yield, which means that your reaction's percent yield will be smaller than 100%.