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Qualification => Subject Doubts => IGCSE/ GCSE => Sciences => Topic started by: AN10 on September 09, 2009, 05:27:09 pm

Title: chem ques
Post by: AN10 on September 09, 2009, 05:27:09 pm
can anybody plz solve this ques for me...i thnk im solving it wrong because my ans is weird (in decimals)

A hydrated aluminium sulfate, Al2(SO4)3. xH2O, contains 8.20% of aluminium by mass. Find the value of x.
Title: Re: chem ques
Post by: Ghost Of Highbury on September 09, 2009, 05:37:32 pm
ok...i too am getting a wierd answer

mss of elements - Al - 17

S - 32
O - 16
H - 1

mass of elements in the hydrated salt

Al- 54
S - 96
O - 192
and water = 18x

==>

first find the total mass

54 / total *100 = 8.2

5400/ total = 8.2

total = 5400 / 8.2 = 658.53

therefore 54 + 96 + 192 + 18x = 658.53

18x = 316.53

x = 17.585
Title: Re: chem ques
Post by: AN10 on September 09, 2009, 05:58:22 pm
i got da same ans....anywayz Thanks!!
Title: Re: chem ques
Post by: Ghost Of Highbury on September 09, 2009, 06:02:24 pm
Quote from: AN10 on September 09, 2009, 05:58:22 pm
i got da same ans....anywayz Thanks!!
welcome..
.
is the question correct??
i mean..is it exact 8.2%
Title: Re: chem ques
Post by: nid404 on September 09, 2009, 07:04:55 pm
 u write the decimal as a whole numbers...so this becomes 18 H2O
never leave ur answer in decimals for such questions...always round it off to the nearest whole no.
Title: Re: chem ques
Post by: Ghost Of Highbury on September 09, 2009, 07:06:50 pm
Quote from: nid404 on September 09, 2009, 07:04:55 pm
u write the decimal as a whole numbers...so this becomes 18 H2O
never leave ur answer in decimals for such questions...always round it off to the nearest whole no.
ohh...thanks!

but is that answer and method correct?
Title: Re: chem ques
Post by: nid404 on September 09, 2009, 07:13:54 pm
Quote from: eddie_adi619 on September 09, 2009, 07:06:50 pm
Quote from: nid404 on September 09, 2009, 07:04:55 pm
u write the decimal as a whole numbers...so this becomes 18 H2O
never leave ur answer in decimals for such questions...always round it off to the nearest whole no.
ohh...thanks!

but is that answer and method correct?

yeah absolutely