CHEMISTRY help?? MOLE question?

1.
Get your periodic table out, and find the atomic masses (/weights) for N (nitrogen); H (hydrogen); S (sulphur/sulfur) and O (oxygen).
For example: for oxygen you should get 16.
To get the mass of one mole of ammonium sulphate you need to add these masses together in various quantities.
Start with the NH4 portion of the molecule, multiply the atomic mass you got for H by four and then add your atomic mass for N to it. Now, multiply this number by 2 because it's (NH4)2. It's the same thing for the sulphate portion, atomic mass of O, multiplied by 4 and then add the mass for sulphur.
Now add the value for the (NH4)2 to the value you worked out for SO4. There you go , you should now have the mass of one mole of Ammonium Suphate ((NH4)2SO4). The units are Grams.

2.
It might help to write the equation this way:
1 H2SO4 + 2 nh4oh ---> 1 (NH4)2SO4 + 2 h2o
(the important bits are capitalised now, and the number of moles of Sulphuric acid and Ammonium Sulphate are also included -usually you wouldn't bother writing "1", but this makes it clearer)
Looking at the equation, you see that there is one mole of H2SO4 on the left hand side and one mole of (NH4)2SO4 on the right. So if you changed it to 0.2 moles H2SO4, to keep things balanced you should also change the right hand side to 0.2 moles (NH4)2SO4.
So you know that you can make 0.2 moles of ammonium sulphate when you use 0.2 moles of sulphuric acid.
But, you are asked for the Mass in the question, so multiply your answer from question 1 by 0.2 and you'll have it.

3.
Your answer from question 2 is the most ammonium sulphate you could make with 0.2 moles H2SO4, the amount you (or the student in the question) actually got is probably less that that.
To get the percentage yield, take the 15.4g the student made and divide it by your answer from question 2. Take this number and multiply it by 100. The answer you get is the percentage yield.

Good Luck.

1.
Mass of one mole of (NH4)2SO4 = 132.14 g/mol
2.
The ratio between H2SO4 and (NH4)2SO4 is 1 : 1.
From 0.2 moles of H2SO4 we obtain 0.2 moles of ammonimu sulfate.
Mass ammonium sulfate = 0.2 mol x 132.14 g/mol = 26.4 g
( 100 % yield)
3.
% of ammonium sulfate = 15.4 x 100 / 26.4 =58.3%