Molarity?................?

Question

i have several problems connected to molarity and don't know how to solve them.it's not necessery to solve all of the problems just if u could explain the general idea how to solve such problems and please write it in details.



1. The density of solution prepared by dissolving 5.46g of copper nitrate in 22.0 ml of water is 1.19g/ml. Calculate the mass of solute in 15ml of solution?


2. Calculate the molarity of a solution prepared by dissolving 9.43g of Na2CO3 on 150ml of water and then diluting to a final volume of 250ml.

3. How many ml of 1.54M H3PO4 hold 0.249 moles?

4. Calculate the moles of sodium hydroxide in 750ml 1.23M NaOH.

thanks very much for the help

Answer 1

The first problem is pretty straight forward...
If the solution is 1.19 grams per milliliter (g/ml), then the mass of 15 ml of solution is 1.19 x 15 = 17.85.

The other questions are much more difficult and involve the use of Avogadro's number (6.023 x 10^23) of the mole. See this site for a good explanation of how to proceed:

http://www.shodor.org/unchem/basic/stoic/index.html

Answer 2

1.
Multiply density x volume of sol'n = mass /g

2.
Calculate the Mr(Relative molecular mass) of Na2CO3
Find moles(Na2CO3)by mass divided by Mr.
This will be the moles per 250 ml, or moles per 1/4 litre.
Multiply moles and 250 ml by 4 to find moles per litre.

3.
Use the equation
moles = [conc] x vol/1000 and rearrange.
vol = moles x 1000/[conc]

4.
Use the same equation
moles = [conc] x vol / 1000