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Chem: working out purity

today we had a evaluation exam

we were given a set of results and some data and asked to compare the % purity of a acid (which the manufacture said it was 99.1%)

i had no clue wat do to

the equation was;

HA+NaOH=NaA+H2O

Average titre was 23.27cm^3
the NaOH was 0.100M, there was 7.1g of substance added to a 250cm^3 volumetric flask and 25cm^3 was taken out via a pipette.

How do I find out the purity ? and how do I compare it with the manufacture purity?
Reply 1
Avatar for C4>O7
C4>O7
5
To work out the purity you need to know the R.F.M. of the 'HA'
Do you mean hydrogen astatide (HAt)?!
Reply 2
Avatar for papz_007
papz_007
OP
no m8.. HA is just an unknown acid , yeah work out the purity of the unkwown acid HA, then compare my answer to the manfacture result
Reply 3
Avatar for C4>O7
C4>O7
5
You do need to know the R.F.M.- otherwise all you can find is the number of moles in 7.1g.
Are you completely sure you havn't missed anything?
Btw HA(s) is not an acid until its HA(aq)
Reply 4
Avatar for papz_007
papz_007
OP
sorry yeah it was 169 the RFM
Reply 5
Avatar for C4>O7
C4>O7
5
mol = conc.vol (dm^-3) = 0.002327 mol NaOH --> 0.002327 mol HA --> 0.02327 mol HA in original 7.1g sample
mass = mol x R.F.M. = 3.933g
purity = 3.933/7.1 x 100 = 55.4% (3s.f.)
Reply 6
Avatar for papz_007
papz_007
OP
is that all the workin u need to do?? Why did u time st he mol of NaOH by 10?

The % is so low compared to the manufacture ... can this be ?
Reply 7
Avatar for C4>O7
C4>O7
5
Because I calculated x moles in 25cm^3 so I assume there was 10x moles in 250cm^3
Yes its quite low. Is it a real sample from a real manufacturer claiming 99.1% purity?
Reply 8
Avatar for papz_007
papz_007
OP
how do i compare my purity value with the manufactures one in a % format?
Reply 9
Avatar for C4>O7
C4>O7
5
I think you just have to comment.
Reply 10
Avatar for papz_007
papz_007
OP
it said that I have to find the % difference m8?


my I got told to do it like this;

99.1-55.4=ans/99.1*100= % difference of purity?

I took away the manufacture purity from the one I worked out, then divded it again by the manufacturn purity then times by 100 to get the difference in purity!
Reply 11
Avatar for Ben.S.
Ben.S.
11
papz_007
it said that I have to find the % difference m8?


my I got told to do it like this;

99.1-55.4=ans/99.1*100= % difference of purity?

I took away the manufacture purity from the one I worked out, then divded it again by the manufacturn purity then times by 100 to get the difference in purity!

That will give you the error in their statement, in terms of a percentage of their stated value (so, around 50-ish). So, you've probably done it right.

Ben

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