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Heinemann questions i had trouble with

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purple_rose:
1. What mass of barium chloride(CaCl2) will remain after 15g sample of the hydrated salt BaCl2.2H20 is heated to drive off all of the water?  (The answer is 12.8g, i keep getting 12.4g)

2. A .5g sample of sodium sulfate (Na2SO4) and .5g sample of aluminium sulfate (Al2(SO4)3 ) were dissolved in a volume of water and excess barium chloride added to preciptate barium sulfate. What was the totoal mass of barium sulfate produced?

(i keep getting 1.02g for this one but the answer is 1.85g)

3. 1. The chlorine in a .63g sample of chlorinate pesticide, DDT (C14H9Cl5), is precipitated as silver cloride. What mass of silver chloride is formed?

4. A precipitate of Fe2O3, of mass 1.43g, was obtained by treating a 1.5L sample of bore water. What was the concentration of iron, in molL-1, in the water?  (the answer is 1.012molL-1, but i keep getting 1.013molL-1, i noe it seems pedantic but i don't noe y it's higher)

Collin Li:
Question 1



(The comes from the molar mass of water)

How are you doing it?

purple_rose:
oic, i must of messed up the molar masses, thanks

Collin Li:
Question 2

Find out the amount of sulfate ions present:



(The comes from the molar mass of a sulfate ion)



I kept all the digits in my calculator. The next decimal places were: "1.84403..." so it's easy to see how the book was off by 0.01. They probably rounded off their calculations too early, or perhaps they used more accurate molar masses - I always use 1 decimal place, because that is how VCAA does it. A difference not worth worrying about.

Similarly, I would not bother with the difference you got in Question 4. The people who write the solutions in the books tend to round off their calculations in the middle, which reduces the accuracy of their final answer.

Collin Li:
Question 3

These questions should be easy (similar to Question 1). You should be using the fact that elements are not created or destroyed (unless it is a nuclear reaction).

This means you can say that the amount of chlorine atoms in DDT is equal to the amount of chlorine atoms in silver chloride.



I'll let you have a go at the numbers. By uppercase "M", I mean the molar masses.

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