Unit 2 chem help

[Zahta]

hey i need help with unit 2 chemistry please.
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

[Lasercookie]

You should be getting the same answer for both if you use the proper SI units.
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.