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Answer to Question #57382 in Other Chemistry for panos

Question #57382
A student titrates a sample with HCl solution using methyl red as indicator and finds it contains 20% K2CO3. Later he realises his sample contained Na2CO3 instead of K2CO3. What is the % concentration of Na2CO3 in the sample?
Expert's answer
According to the given conditions the sample can contain the same number of moles of K2CO3 or Na2CO3.
Let’s assume that he has 100 g of the sample. Then
m(K2CO3) = (20% × 100 g)/100% = 20 g
At the same time the amount K2CO3 is: ν(K2CO3) = m(K2CO3)/M(K2CO3) = 20 /138 mol = 0.1449 mol
When he realizes the he has got Na2CO3:
ν(K2CO3) = ν(Na2CO3) =m(Na2CO3)/M(Na2CO3) = 0.1449 mol
m(Na2CO3) = 0.1449 mol × 106 g/mol = 15.36 g
And, finally, the concentration of Na2CO3 is:
W(%) = m(Na2CO3)×100%/100 g = 15.36 %

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