Heinemann questions i had trouble with

[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.

[purple_rose]

um....for question 2 why is there a +3?

[Collin Li]

It's 3 multiplied by the last fraction, because that last fraction represents the amount of aluminium sulfate, but there are 3 sulfate ions per aluminium sulfate.

[purple_rose]

For question 3, is it possible to write an equation for wot is going on?

So this is how u do it, correct me if i'm wrong
1. so u find the mole of DDT which is .00356
2. you then times by 5 becos theres 5 Cl which is .017797
3. you then times by M of silver chlorine (which is 143.32) to get the mass of AlCl, i got .08898, but the back of says the answer is 1.3g

"because that last fraction represents the amount of aluminium sulfate"...... i don't understand wot u mean

[Collin Li]

Yeah, what you said in words is spot on, but I think your numbers are wrong:

Using:

[Collin Li]

"because that last fraction represents the amount of aluminium sulfate"...... i don't understand wot u mean

This is:



The fractions in the top equation with numbers are just the masses divided by the molar masses (which give you the amount of moles).

[purple_rose]

ooooooooooooooooooooooooooo. i noe wot i did wrong, i timed it by 5 twice,
i get it now, awesome


thanks coblin

[Collin Li]

No problem

[lanvins]

More heinemann question help- Mostly half equations questions i had trouble with

1. 2S2O4 -2(aq) + H20(l) = S2O3 -2(AQ) + 2HSO3 - (aq)

Write the ionic half equations for the oxidation and reductions that occur when sodium dithionite is mixed with water

2. In dry cells commonly used in torches, an electric current is produced from the reaction of zinc metal with MnO2. During this reaction, Zn2+ ions and Mn2O3 are formed.

Calculate the mass of zinc that would be needed to react completely with 8.g of MnO3 in a dry cell.
I got 6g but thats wrong according to the book

3. The thermite process can be used to weld lengths of railway track together. A mould placed over the ends of the rails to be joined is filled with a charge of aluminium powder and iron(iii) oxide. When the mixture is ignited, a redox reaction occurs to form molten iron, which joins the rails together.
a. Write a half equation for the conversion of iron(iii) oxide to metallic iron.
i got Fe2O3 + 6H+ +6e- = 2Fe + 3H20- but thats wrong according to the book

b. Write the overall equation for the thermite process.

[Mao]

1)

reduction:


oxidation:



2) the equation will be:



i'm assuming you meant 8.0g of MnO₂:







3) In the mould, there are no hydronium ions present. In fact, the reaction does not involve (or occur in the presence of) water. So in this case, the oxygen is balanced by instead of

a)

b)

[lanvins]

yea i get that....so ur allowed to use S204-2 twice?....i thought i had to use the water