acids and bases question

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A*my
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#1
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#1
nitric acid, HNO3, and nitrous acid, HNO2 , are 2 bronsted-lowry acids containing nitrogen. a student measures the pH of 0.045 mol dm-3 solutions of HNO3 and HNO2. (pKa = 3.35) and found the acids had different pHs.

a) why are the pH different?
b) calculate the pH value of 0.045 mol dm-3 HNO3
c) calculate the pH value of 0.045 moldm-3 HNO2


I can't answer these questions? with parts b and c, I don't know how i'm meant to change the equation
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Giovannii
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#2
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#2
1) HNO3 is a strong acid so it fully dissociates H+ ions solution while HNO2 will partially dissociate H+ ions in solution.
2) In a strong acid [H+] = [HA] so you will only need to -log (H+) to get the pH
3) For a weak acid first, you work out Ka by 10^pKa. Then using that value, H+^2 will be ka x [HA] but pH is -log(H+). So you would need to square root H+ then -log it

Hope this helps
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A*my
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#3
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#3
(Original post by Giovannii)
1) HNO3 is a strong acid so it fully dissociates H+ ions solution while HNO2 will partially dissociate H+ ions in solution.
2) In a strong acid [H+] = [HA] so you will only need to -log (H+) to get the pH
3) For a weak acid first, you work out Ka by 10^pKa. Then using that value, H+^2 will be ka x [HA] but pH is -log(H+). So you would need to square root H+ then -log it

Hope this helps
ah I see! thanks. I wasn't aware HNO2 was a weak acid :/
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A*my
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#4
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(Original post by Giovannii)
1) HNO3 is a strong acid so it fully dissociates H+ ions solution while HNO2 will partially dissociate H+ ions in solution.
2) In a strong acid [H+] = [HA] so you will only need to -log (H+) to get the pH
3) For a weak acid first, you work out Ka by 10^pKa. Then using that value, H+^2 will be ka x [HA] but pH is -log(H+). So you would need to square root H+ then -log it

Hope this helps
I just had a look at the OCR A spec to try and work out if there were any weak acids i need to learn off by heart. I couldn't see any. I only know things like CH3COOH is weak.
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Giovannii
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#5
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(Original post by A*my)
I just had a look at the OCR A spec to try and work out if there were any weak acids i need to learn off by heart. I couldn't see any. I only know things like CH3COOH is weak.
In the OCR A spec they mostly want you to know that:
HCl, HNO3, H2SO4, HBr, HI, and HClO4 are all strong acids.
Most other acids will be weak acids they will give it away that it is a weak acid by giving you a Ka or pKa.
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Amethyst190
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#6
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#6
(Original post by Giovannii)
1) HNO3 is a strong acid so it fully dissociates H+ ions solution while HNO2 will partially dissociate H+ ions in solution.
2) In a strong acid [H+] = [HA] so you will only need to -log (H+) to get the pH
3) For a weak acid first, you work out Ka by 10^pKa. Then using that value, H+^2 will be ka x [HA] but pH is -log(H+). So you would need to square root H+ then -log it

Hope this helps
how do you get the [H+]
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ran-dumb
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#7
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#7
For strong acids like HNO3, concentration of acid=[H+] because we assume complete dissociation
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