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AQA A2 CHEM 4 Acids and Bases - Confused

Hi,

Ive been doing some past papers on Acids and Bases and I always thought that square brackets
"[ ]" around OH- or H+ meant the concentration of...

How come for some questions I need to work out the moles to find out the concentration?

For example, for [OH-] I derive the number of moles from the volume and concentration given n=CV

I work out [H+] from Kw/[OH-] and then work out the pH

I use the number of moles for [OH-] and get the correct answer. But when I use the concentration of the sodium hydroxide the answer is completely different.

Please help, so if the question involves using [OH-] or [H+] do I substitute them useing the number of moles or concentration?
Original post by MickoXD
Hi,

Ive been doing some past papers on Acids and Bases and I always thought that square brackets
"[ ]" around OH- or H+ meant the concentration of...

How come for some questions I need to work out the moles to find out the concentration?

For example, for [OH-] I derive the number of moles from the volume and concentration given n=CV

I work out [H+] from Kw/[OH-] and then work out the pH

I use the number of moles for [OH-] and get the correct answer. But when I use the concentration of the sodium hydroxide the answer is completely different.

Please help, so if the question involves using [OH-] or [H+] do I substitute them useing the number of moles or concentration?


If everything is in the same solution then the volume is constant

... so as concentration = mol/volume

then you can work in moles
Original post by MickoXD
Hi,

Ive been doing some past papers on Acids and Bases and I always thought that square brackets
"[ ]" around OH- or H+ meant the concentration of...

How come for some questions I need to work out the moles to find out the concentration?

For example, for [OH-] I derive the number of moles from the volume and concentration given n=CV

I work out [H+] from Kw/[OH-] and then work out the pH

I use the number of moles for [OH-] and get the correct answer. But when I use the concentration of the sodium hydroxide the answer is completely different.

Please help, so if the question involves using [OH-] or [H+] do I substitute them useing the number of moles or concentration?


[ ] is strictly concentration, so you must have gotten the right answer by substituting a concentration whilst thinking it was a molar value

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