# Finding pH given molarity and volume

## Homework Statement

If 50ml of 0.004M H2CrO4 was combined with 50 ml of 0.005M HNO3 what is the pH of the resulting solution?

## Homework Equations

Moles H3O=Moles OH ?
Formula for KA

## The Attempt at a Solution

So, here's what I have so far.

I made the H2CrO4 represent the acid, and the HNO3 represent the base. I then subtracted them to get the excess, and then found the molarity of that. But I have no idea if I chose the right things to represent acids and bases, in fact I have no idea if I'm doing the right thing. Somebody please help!

Redbelly98
Staff Emeritus
Homework Helper
Both of these are acids, there is no base here.

Also, do these substances dissociate completely, or must Ka be taken into account?

p.s. Welcome to Physics Forums

Last edited:
I believe HNO3 is a strong acid- you can assume that dissociates completely. However, the other acid is NOT a strong acid--you will need Ka for that.

BUT. I'm assuming since the Ka is very small, adding the H+ ions from that species will not change the # of H+ you get from HNO3 because of sig figs.

Therefore, .05L x .005M H+ (from the complete dissociation of HNO3) = 0.00025mol H+

next, find the molarity. 100ml total = .1L
.0025 Molar H+
-log .0025 = 2.6 = pH

Last edited:
Borek
Mentor
the Ka is very small

No.

.0025 Molar H+

No.

-log .0025 = 2.6 = pH

No.