please help me do these equation problems..??

Question

1)3NO2 (g) + H2O (l)>>>NO (g) + 2HNO3(aq) .How many mol of water is needed to react w/8.4 mol NO2?
2)P4(s) + 6F2 (g)>>4PF (g) .How many grams of fluorine is needed 2 react w/6.20g of phosphorus?
4)according to the balanced equation for the combustion of ethane, C2H6, 2C2H6(g) + 7O(g)>>4CO2(g) + 6H2O(l), for every molecule of ethane that reacts, CHOOSE RIGHT ANSWR:
(A)2 molecules of carbon dioxide r produced (B) 3 molecules of water is produced (C)A&B D)neither
68) write formula for each compound
A)aluminum carbonate
B)silicon dioxide
C)potassium sulfide
D) manganese(II)Carbonate
E) sodium bromide

Answer 1

1) you need 1 mol of H2O for every 3 mol of NO2 (from the 3NO2 + H2O), so for 8.4 mol of NO2, you need 8.4/3 = 2.8 mol of H2O

2) Same idea as question 1: you use 6 mol of F2 for every mol of P4. The molecular weight of P4 is 30.974 * 4 = 123.896 g/mol. The molecular weight of F2 is 18.998 * 2 = 37.996 g/mol. 6.20 g of P4 is (6.20 g / 123.896 g/mol) = 0.0500 mol, so you need .0500 mol * 6 of F2, which is 0.300 mol. The number of grams of F2 can then be found as 0.300 mol * 37.996 g/mol = 11.4 g of fluorine.

4) 1 molecule of C2H6 will react with 3.5 molecules of O2 to produce 2 molecules of CO2 and 3 molecules of H2O. (divide the equation by 2). That means that B is correct.

68)
A) Al2(CO3)3 (Al is +3, CO3 is -2 charge)
B) SiO2 (Just from the name: since silicon does not have a prefix, it means 1, di- means 2 oxygens)
C) K2S (K is +1, S is -2 charge)
D) MnCO3 (Mn's charge is given - (II) means +2, and CO3 is -2 charge)
E) NaBr (Na is +1, Br is -1 charge)