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April 21, 2014

April 21, 2014

Posted by **Franky** on Sunday, October 16, 2011 at 6:01am.

a) How many moles of sodium hydroxide are there in 19.00 cm3 of 0.100 mol dm3?

My answer: 0.0019 mol

b) How many moles of hydrochloric acid are, there in 25.00 cm3 of the acid solution?

My answer: 0.0019 mol

c) How many moles of hydrochloric acid are, therefore in 1000cm3 of solution?

My answer (this is the one I'm unsure about): 0.0076 mol

I did 0.0019 x 1000/25.00

d) What is the concentration of the hydrochloric acid solution?

My answer: 0.076 mol dm3

I am stuck on this though:

The concentration of hydrogen ions in a 0.1 mol dm3 solution of phosphoric acid, H3PO4?

I would be able to do it if I knew the correct equation that = H3PO4 but I can't work it out.

If any of these are wrong can you please explain how they should be done thank you :)

- Chemistry -
**DrBob222**, Sunday, October 16, 2011 at 1:31pmYou don't have enough information to help you.

a is right.

b--What acid solution? If this is a titration the answer is ok.

c--If this is NaOH titrated with HCl both c and d are correct.

For the last (un-lettered) question you have, H3PO4 is a triprotic acid. k1 = about 10^-3, k2 = about 10^-8 and k3 = about 10^-13.

So you set up an ICE chart and calculate the concn H^+.

............H3PO4 ==>H^+ + H2PO4^-

initial.....0.1.......0......0

change......-x........x.......x

equil......0.1-x......x........x

Essentially you ignore k2 and k3 since they are so much smaller than k1.

k1 = (H^+)(H2PO4^-)/(H3PO4)

Look up k1, subtitute the ICE data into the k1 expression and solve for H^+ = x.

- Chemistry -
**Franky**, Sunday, October 16, 2011 at 2:56pmThank you. I know, the question was very long so I tried to shorten it for this but left out some details sorry!

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