Posted by Franky on Sunday, October 16, 2011 at 6:01am.
Can you check if this is correct please?
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 1:31pm
You don't have enough information to help you.
a is right.
bWhat acid solution? If this is a titration the answer is ok.
cIf this is NaOH titrated with HCl both c and d are correct.
For the last (unlettered) 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......0.1x......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 2:56pm
Thank you. I know, the question was very long so I tried to shorten it for this but left out some details sorry!