Question

How much solute is dissolved in 31 ml of a sodium hydroxide solution with a pH of 13.3576?

Answer 1

@dtan5457

Answer 2

@matt101

Answer 3

If you have the pH, you can find the pOH by subtracting the pH from 14. The pOH is similar to pH, just in terms of [OH-] rather than [H+]. Then answer the following questions and you'll have the answer to yours!

What would the pOH of the solution be?
What would the corresponding [OH-] be?
How much OH-, in moles, are there?
How much NaOH (the solute), in grams, was originally dissolved?

Answer 4

Right so the pOH is 0.6424 but what do you mean corresponding OH-?

Answer 5

@matt101 I'm confused

Answer 6

How do you go from pH to [H+]?

It's the same process to go from pOH to [OH-]

Answer 7

Wouldn't I do -log? @matt101

Answer 8

That's going from [OH-] to pOH. To go from pOH to [OH-], you need to find 10^(-pOH).

Answer 9

Okay so 10^(-0.6424)=0.228

Answer 10

How would you find moles of that? (Im sorry but pH confuses me a bit)

Answer 11

What you've just found is that [OH-]=0.228 M.

You're given volume of solution in the question, so using this information you can find moles of OH- in the solution!

Answer 12

Is there a formula for doing so?

Answer 13

Do you remember the formula for finding concentration?

Answer 14

Do you mean M1V1=M2V2?

Answer 15

Sort of - that's for a dilution when you're dealing with a couple different concentrations.

I'm talking about just one half of that equation: MV=n, or M=n/V, where M is the concentration, n is the moles of solute, and V is the volume of solution.

Answer 16

Oh alright, so we are looking for volume of the solute? so can we just plug in the values we found?

Answer 17

Yup! And we're finding moles, because we have concentration and volume already.

Answer 18

Oh alright, so MV=n
(0.228M)(31mL)=7.068
And thats the answer?

Answer 19

Well remember the equation using volume in L, not mL, so it'll be (0.228M)(0.031L)=0.007068 moles.

That's how many moles of OH- are in solution, meaning that's how many moles of NaOH were dissolved (because each molecule of NaOH contains 1 OH-).

If you need the answer in grams, it's a simple conversion from moles to mass!

Answer 20

Could you please show me the conversion? I just want to make sure I understand

Answer 21

Like just use stoich?

Answer 22

Oh I know so multiply the 0.007068 by the molar mass which is 40 so the answer is 0.28272g

Answer 23

Absolutely right!