chemistry class
posted by tere on .
A flask is charged with 0.124 mol of A and allowed to react to form B according to the reaction A(g) → B(g). The following data are
obtained for (A) as the reaction proceeds:
Time (s) 1 10 20 30 40
Moles of A 0.124 0.110 0.088 0.073 0.054
1) The average rate of disappearance of A between 10 s and 20 s is __________ mol/s.
answer: 2.2 × 103
2) If the rate law for the reaction
2A + 3B → products
is second order in A and first order in B, then the rate law is rate = __________.
answer: k(A)2(B)
i know the answers but how do i figure problems like the above? i have a bunch of similar problems to figure out.

10 sec, 0.110
20 sec, 0.088
0.1100.088 = 0.022 moles/10 sec = 0.0022 moles/sec.
rate constant = k[A]^{x}[B]^{y}
where x is the order of A and y is the order of B and [A] and [B] are the concentrations, in moles/L, of the reactants. Except for elementary reactions, the order CAN NOT be determined from the coefficients in the balanced equation. x and y must be determined experimentally. 
thank you i kept getting 0.022 because i didn't divide by time.