This seems like a simple enough question, and I'm usually good at these... I don't know what I'm doing wrong this time. Probably making a stupid mistake. My answer was 18.67g, and if you divide that by 2, you get approx. 9.33g.. which as you said is the correct answer.
My method: Molar ratio of (NH4)2SO4: NaOH = 1: 2
Mr of (NH4)2SO4: 2(14+4x1) + 32 + 16x4 = 132 g/mol
Number of moles of (NH4)2SO4 according to the given mass of (NH4)2SO4: 15.4/132 = 0.1167 mol
As the molar ratio of (NH4)2SO4 to NaOH is 1:2, the number of moles of NaOH are: 2 x 0.1167 = 0.233 mol
Therefore, mass of NaOH required: Mr(of NaOH) x mol(of NaOH), which is: 2(23+16+1) x 0.233 = 18.7 g.... did I do something wrong? Because you said the answer is 9.32g.. which is the half of 18.7g... You were the one confused, but I`m in the same boat as you now. lol any help would be appreciated.