ph question

[fbradley99]

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I know this is probably a very simple ? but I think i am putting too much thought into it. Can someone help?

what is the pH of a solution containing 5mM H2S?

The answer is 2. Where are they getting H+= 1X10-2???

What does mM mean? I dont remember that from chemistry

 

[G1SG2]

I know this is probably a very simple ? but I think i am putting too much thought into it. Can someone help?

what is the pH of a solution containing 5mM H2S?

The answer is 2. Where are they getting H+= 1X10-2???

What does mM mean? I dont remember that from chemistry

mM=milli molar

That's a weak acid. You need to find out how much dissociates by setting up the ICE table (initial, change, equilibrium). You also need the Ka, or acid dissociation constant.

 

[matth87]

5mM = 5x10^(-3) molar
It's just the mill prefix.

pH = -log(H+)
pH = -log(5x10^(-3))

In this case since its 5x10-3 the pH is between 2 and 3 since the number is between 1x10^(-2) and 1x10^(-3).

 

[G1SG2]

5mM = 5x10^(-3) molar
It's just the mill prefix.

pH = -log(H+)
pH = -log(5x10^(-3))

In this case since its 5x10-3 the pH is between 2 and 3 since the number is between 1x10^(-2) and 1x10^(-3).

H2S is not a strong acid.

 

[matth87]

H2S is not a strong acid.

good point.

So yeah you would need Ka then you could set up

Ka = [H+][A-]/[HA]

If you know Ka it would be

[HA]Ka = x^2 then you could find H+ conc.

 

[fbradley99]

That's all that was given in the problem. The answer is ph=2. Its from the new Kaplan series books. I knew I was putting too much thought into it! It follows the trick 🙂