Question

Molybdenum (Mo) is a heavy metal that can have severe health effects on humans and animals. Tailings from a mining operation in Colorado have polluted surface waters used by grazing animals with Mo. Based on the following information determine the concentration of the three Mo species in the stream. [Mo]T = 5.1x10^-4 M pH = 4.20

H2MoO4 (aq) = HMoO4^1- (aq) + H+ (aq) pKa1 = 4.0

HMoO4^1- (aq) = MoO4^2- (aq) + H+ (aq) pKa2=4.24

Answer 1

What does [Mo]T mean?

Answer 2

Have you determined fractions of a polyprotic acid such as H2A?
Here is the denominator which I will call D,
D = (H^+)^2 + k1(H^+) + k1k2. You know H^+ from the pH. You know k1 and k2. Solve for D and use it below.
fraction of H2A = (H^+)^2/D
fraction of HA^- = k1(H^+)/D
fraction of A^2- = k1k2/D
Then fraction of each specie times initial M = molarity of each specie.
Final check. Sum of species should equal Total at start. Post your work if you have questions about this.