In which of the following reactions is δs positive? select one: a. 2no2 (g) → n2o4 (g) b. baf2 (s) → ba2+ (aq) + 2f- (aq) c. co2 (g) →co2 (s) d. 2h2 (g) + o2 (g) →2h2o (g)

letter b. 

Why?
 Because Entropy(ΔS) INCREASES when INCREASING disorder in a reaction.

A. INCORRECT. Negative ΔS. This is because there are less molecules of gas in the products compared to the reactants.
B. CORRECT. Positive ΔS. This is because you are increasing disorder by breaking down a solid(highly ordered) molecule into two ions in solution(in solution they are less ordered).
C. INCORRECT. Negative ΔS. This is because you are going from a (g)gas which has more disorder to a (s)solid which has much less disorder than a gas.
D. INCORRECT. Negative ΔS. This is because you are decreasing disorder by building a single molecule. There are also more moles of gas on the reactants side indicating a negative ΔS.

Please comment with any further questions. Hope this helped!

About 8 gallons of fuel will be used. (8.08)

Maybe you should get some from your teacher and learn more so you can get your grades up and maybe study a bit more