Couple of questionz.

[username]

1. Calcium chromate CaCrO4 is only slightly soluble in water. It dissolves according to: CaCrO4 <-> Ca(2+) + CrO4(2-) to produce a yellow solution. An equilibrium mixture is prepare consisting of a few crystals of calcium chromate in CrO4(2-). When a small volume of concentrated CaCl2 is added to the mixture, the expected change would be -->

a. Crystals would dissolve, the solution becomes lighter
b. Crystals would dissolve, the solution becomes darker
c. Crystals would form, the solution becomes lighter
d. Crystals would form, the solution becomes darker

Main question is, in these instances, how do you know which side of the mixture they add the thing to? Cuz it could be the product mixture (backward reaction) or the reacting mixture (forward reaction) neh? (The answer is C btw)


2. Oxygen gas was bubbled into an acidified solution containing sodium chloride and sodium iodide. The solution had a concentration of 1M, the gas was at 1am pressure and the temp at 25. If the experiment was conducted for five minutes, which would be expected to decrease?

a. The Cl- concentration of the solution
b. The I- concentration of the solution
c. The Na+ concentration in the solution
d. the pH of the solution

So far I've got the two equations:

O2 + 4H+ + 4e <--> 2H20
2I- <--> I2 + 2e

Now what? >_< (The answer is B btw)

Thanks! 

[Mao]

1. CaCl2 dissolves to Ca2+ and Cl-, where Ca2+ is on the products side, pushing the system to the left

2. so far so good, the reactions are right hence we can cross out a and c (they don't react)
as the reaction proceeds forwards, H+ is consumed, [H+] decreases, hence the pH increases, (not D)
I- is consumed, hence [I-] decreases, B

[username]

Thanks Mao! So um, when they say 'add to mixture', do they mean 'add to mixture already at equilibrium' or 'reacting mixture'.

[Mao]

Thanks Mao! So um, when they say 'add to mixture', do they mean 'add to mixture already at equilibrium' or 'reacting mixture'.

the question already told you it was at equilibrium: "An equilibrium mixture is prepare consisting of a few..."