Post by: Zahta on October 20, 2011, 09:03:05 pm
Their is a question which says 30.0 mL of 0.100 M HNO3 is added to 50.0ml water. what is the ph of the diluted soultion
However im confused because in the text book example they use the formula n=c x v
then they got the total voulme of 30.0+50.0 =80.0
then used c=n/V
then calcultaed the pH
yet why cant you use the C2=V1 x C1 / V2 which is the diulted solution, what wording is diffierent . and how would i know which formula to use
Yet in this different question
which says Calculate the pH of the following mixtures
10 mL of 0.025 M HCl is diluted to 50 M of solution , the textbook uses the C2=V1 x C1 / V2 solution
Post by: Lasercookie on October 20, 2011, 09:28:29 pm
Technically with what you described, you are (probably) getting the same answer for both, but the answers are in different units.
The first example
n is in mol, concentration is in mols per litre (also called Molars) and volume is in litres.
Add in the 50mL (0.050L) of water - dilutes the solution which changes the concentration:
Effectively, we have:
But because the amount of moles of the solute (HNO3) doesn't change, we can derive this formula
Solve for c_2:
This is an example on why I always try to stick with the proper units.
I suspect a similar situation for the second problem. When something is diluted, the amount of solute stays the same (n) so you should always be able to use both formulas - just pick the one that fits with the numbers you are given easier.