Need help with chemistry calculations.?

There are some word problems that I did various calculation on, but cannot seem to get to the given correct answers. I need to know what am I missing when doing calculations. 1) If the actual yield of iron metal was 68.2 moles, what is the percent yield of the reaction. Answer: 75 percent The EQ: Fe2O3 (s) + 3 CO... show more There are some word problems that I did various calculation on, but cannot seem to get to the given correct answers. I need to know what am I missing when doing calculations.
1) If the actual yield of iron metal was 68.2 moles, what is the percent yield of the reaction. Answer: 75 percent
The EQ: Fe2O3 (s) + 3 CO (g) yields 2Fe (s) + 3 CO2 (g). 7.24 kg of iron ore filled in a small blast of furnace.

2) Calculate the volume of 0.18M hydrochloric acid needed theoretically to react with 20.0 ml of the 0.10M sodium carbonate solution
EQ: Na2CO3(aq) + 2HCl (aq) yields 2NaCl (aq) + H2O. Answer: 27.8

3) Ammonia is formed from the reaction of hydrogen and nitrogen.
Eq: 3H2 (g) + N2 (g) yields 2NH3 (g).
How many grams of which reagent remains theoretically after 120 moles of hydrogen are reacted with 50 moles of nitrogen. Answer: 280g N2.

4) What is the volume of 0.186M H2SO4 is needed to titrate 34.2 ml of 0.320M NaoH in the reaction.
EQ: H2SO4 (aq) + 2NaOH (aq) yields Na2SO4 (aq) + 2H2O (l)
Update: I need help with chemistry calculations.
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