Homework Help: college chemistry - molarity question

Posted by lholmq on Wednesday, April 29, 2009 at 1:23am.

My textbook doesn't show me how to do problems like the following, and I'm having a hard time finding similar problems online. Most of the molarity problems I'm finding involve mixing a substance with water, but these involve mixing moles of substances with moles of other substances and I'm confounded as to how to try to solve them. If anyone can show me steps to solve these two, I would be extremely grateful:

1) If 35 mL of 2.0 M H2SO4 is mixed with 35mL of 1.0 M NaOH, how many moles of H2SO4 remain after completion of the reaction?

2) What is the molarity of Ca(NO3)2 in a solution resulting from mixing 150.0 mL of 0.0200 M HNO3 with 150.0 mL of 0.0100 M Ca(OH)2?

Thank you so much. :)

college chemistry - molarity question - Dr Russ, Wednesday, April 29, 2009 at 7:11am
For this type of problem the usual approach is to start with a balanced equation for the reaction. This will tell you how many moles of each react together.

Then using the using the molarity and volume calculate the number of moles of each component.
For example the number of moles of H2SO4 in the first problem is
(35 x 2.0/1000)moles

Based on the equation and the number of moles of each compound work out which compound is in excess. In Q1 it is the H2SO4 as we are told this in the question, but you should convince yourself this is correct.

for 1) you can the find the number of moles (Mx) of H2SO4 left over (remain). The question does not ask you to calculate the final concentration!

for 2) the compund that is not in excess will determine the number of moles of Ca(NO3)2 in solution (Mc). The new volume is 300.0 ml so the molarity is

Mc/0.3000

Hope this helps.

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