[SportsMan101]

3.  What is the resulting molarity of Na+ ions in a solution prepared by mixing 250 mL of 0.04 M NaCl with 200 mL of 0.02 M Na2SO4?
(1)  0.03 M        (2)  0.04 M      (3)  0.01 M            (4)  0.02 M         (5)  0.05 M

For this problem, what I tried doing was to get the number of moles by multiplying the molarity by the amount of liters. I then added the # of moles together to get .042. The answer is .04 M, but not sure how exactly they got it.

4.  A mixture of butene, C4H8, and butane, C4H10, is combusted in air to give carbon dioxide and water. Suppose you burn 286.56 g of the mixture and obtain 414.46 g of H2O.  What is the mass of butene closest to in the mixture?     C4H8 (g) +  6 O2(g) ®  4 CO2(g) +  4 H2O(g)           2 C4H10 (g) +  13 O2(g) ®  8 CO2(g) +  10 H2O(g)
1)  107 g butene      2) 159 g butene    3) 174 g butene    4) 101 g butene       5) 112 g butene

For this one, I'm not sure how to start it. Would I start with the 414.46 g of H2O and use stochiometry to eventually get to the amount of butene, and I'd multiply the 286.56 g in there?

[DesertRose]

I never did this before, but what i can tell you is this;
for question #3, they got .04 because they simply wrote their answer to 2 decimal places while you wrote yours to 3. So you got the correct answer there. if you had wrote yours to 2 d.p, you'd also get .04M. I hope this helped!

[Borek]

A mixture of butene, C4H8, and butane, C4H10, is combusted in air to give carbon dioxide and water. Suppose you burn 286.56 g of the mixture and obtain 414.46 g of H2O.  What is the mass of butene closest to in the mixture?

If you burn 286.56 g of just butene - how much water will you get?

If you burn 286.56 g of just butane - how much water will you get?

Mass of water linearly depends on the mixture composition. If you will find this dependence, you will easily calculate mass of butene knowing water mass.