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I don't understand the mark scheme

A household bleach contains sodium chlorate (I), NaClO, as its active ingredient. The concentration of NaClO in the bleach can be found by using its reaction with hydrogen peroxide, H2O2. NaClO(aq) + H2O2(aq) --> O2(g) + NaCl(aq) + H2O(l)

a) A student added an excess of aqueous hydrogen peroxide to 5cm3 of the bleach 84cm3 of oxygen gas were released. What was the concentration, in mol dm^-3, of NaClO in the bleach.

This is what I did:
mol of O2 is: n=v/24 = 0.084/24 = 3.5 x 10^-3
concentration of NaClO is: c=n/v = 3.5 x 10^-3/ 5 x 10^-3 = 0.7

b) The label on the bottle of household bleach states that the bleach contains a minimum of 4.5g per 100cm3 of NaClO. Use your anser to decide whether or not the information on the label is correct.

Here's what I did:
n=4.5/74.5 = 0.0604
c=n/v = 0.0604/0.1 = 0.604
The label was incorrect (<-- but apparently this bit is wrong and the answer is supposed to be 'bleach is 10 x 0.0604 = 0.604 mol dm−3 which is less than answer to (a) and therefore label is correct', but I don't understand why)

(edited 10 years ago)

Reply 1

tazmaniac97

anyone? I've got the exam tomorrow :eek:

What is bleach?

Original post by tazmaniac97

b) The label on the bottle of household bleach states that the bleach contains a minimum of 4.5g per 100cm3 of NaClO. Use your anser to decide whether or not the information on the label is correct.

Here's what I did:
n=4.5/74.5 = 0.0604
c=n/v = 0.0604/0.1 = 0.604
The label was incorrect (<-- but apparently this bit is wrong and the answer is supposed to be 'bleach is 10 x 0.0604 = 0.604 mol dm−3 which is less than answer to (a) and therefore label is correct', but I don't understand why)

Highlighted the keyword in the question. If it contains a minimum of 4.5g per 100cm3 of NaClO, it can have more. Since you calculated that the actual concentration is higher than the theoretical concentration at 4.5g/100cm3, the bleach must contain more than 4.5g of NaClO per 100cm3 of bleach, therefore the label is correct.