1. ## pH

Question: What is the pH of a solution containing a mixture of 25.0mL of 0.25mol/L HNO3 and 20.0mL of 0.35mol/L HCl?

So i found the moles of HCl = 0.007, then found the moles of HNO3 = 0.00625.
I then subtracted the moles of HNO3 from the moles of HCl = 0.00075 moles in excess of HCl. This means the moles of the solution will be 0.00075 and be acidic.
I then found the volume of the solution which is 0.025 + 0.02 = 0.045L, which i then used to find the concentration of the solution:
C = 0.00075/0.045 = 0.16666666...
I then plugged it into pH = -log[H+] = 1.77815125 = 1.8.

The problem is that the answer is 0.5. Can someone look over my working and point any flaws in my logic, i'm pretty sure its not a numerical error coz i double checked.

thanks

2. ## Re: pH

Originally Posted by DatAtarLyfe
Question: What is the pH of a solution containing a mixture of 25.0mL of 0.25mol/L HNO3 and 20.0mL of 0.35mol/L HCl?

So i found the moles of HCl = 0.007, then found the moles of HNO3 = 0.00625.
I then subtracted the moles of HNO3 from the moles of HCl = 0.00075 moles in excess of HCl. This means the moles of the solution will be 0.00075 and be acidic.
I then found the volume of the solution which is 0.025 + 0.02 = 0.045L, which i then used to find the concentration of the solution:
C = 0.00075/0.045 = 0.16666666...
I then plugged it into pH = -log[H+] = 1.77815125 = 1.8.

The problem is that the answer is 0.5. Can someone look over my working and point any flaws in my logic, i'm pretty sure its not a numerical error coz i double checked.

thanks
That's an acid and an acid. They don't neutralise! Whoops!

Resultant volume = 0.045L as you already pointed out

Resultant moles of acid = 0.007 + 0.00625 = 0.01325mol

[H3O+] = resultant concentration of acid (both strong acids) = 0.0135/0.045 = 0.3 mol L^-1
pH = -log_10[0.3] = 0.5228787453...

3. ## Re: pH

Oh wow. That was pleb.
Thanks for that

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