how to do concentrations?

Question

A solution is prepared by dissolving 10.6 g ammonium sulfate in enough water to make 100.0 mL of stock solution. A 10.00 mL sample of this stock solution is added to 58.00 mL of water. Calculate the concentration of ammonium ions and sulfate ions in the final solution.

Answer 1

Molar mass of ammonium sulfate:132.14 g/mol
10.6g (NH4)2SO4 in 100.0 mL <==> 1.06g (NH4)2SO4 in 10.00 mL ==> 1.06g (NH4)2SO4 in 68.00 mL <==> 0.00802mol (NH4)2SO4 in 68.00 mL ==>
0.00802 mol/ 0.068L = 0.118M
[SO4--] = 0.118M
[NH4+] = 0.236M