[KavX (OP)]

hi,

for some these might seem simple for me I just cant seem to grasp it.

heres the questions. ive done some of the questions on my sheet thats why the numbers arnt in sequence.

1. If we burn 24g of magnesium in oxygen, how many grams of MgO will we get?
5. What mass of sulfuric acid is needed to reast with aluminium to produce 0.5 moles of aluminium sulfate?
6.Calculate the amount of carbon dioxide obtained from heating 5g of calcium carbonate?
10. When 43.2g of HgO is decomposed, what mass of mercury is produced?(this question troubles me a bit if some1 could explain this plz  it would b great)
13. Calculate the number of moles and the mass of oxygen required to burn 3 moles of hydrogen.
19. Calculate the mass of calcium oxide formed from the decompostition of 0.5kg of calcium carbonae.
20. what mass of sodium chloride is obtained when 53g of sodium carbonate is added to 100g of hydrochloric acid.

well thats it, i understand basic of figuring out some but see like with questions such as 13 i dont understand how to do the working including 19 and 20.

Any help would be great and appreciated. Id prefer working over answers because it would greatly ease my understanding of this.

thx.

[daveshorts]

13.
2 molecules of hydrogen react with one mole of oxygen so
2 moles of hydrogen react with one mole of oxygen

from this you can work out how many moles of oxygen will react with 3 of hydrogen.

you then just multiply this by the mass of a mole of oxygen - remember that there are 2 atoms of oxygen in a molecule.

[Cut Chemist]

For each of those questions, there is a chemical reaction that you need to know in order to solve it.  Once you figure out the chemical reaction its just stoicheometry (unit conversions).

For #13 the chemical reaction is 

H₂ +  O₂  -->  H₂O

but you have to balance it ... so

2H₂ +  O₂  -->  2H₂O

Use that reaction to solve the problem like dave said above.

[DrDick]

The first step, as mentioned, is to determine the balanced chemical equation for the reaction.  Once you've done that, there are generally three steps to a stoichiometry problem:
  1) get to moles of something
  2) use stoichiometry of equation to get to moles of something else
  3) convert from moles to whatever unit you're looking for

One of the "tricks" is knowing how to convert.  Dimensional analysis (using the units algebraically to help you solve the problem) is the easiest tool, if you're familiar with it and can effectively use it.

So, to look at your first problem:
  1)  how many moles of Mg do you have in 24 g of Mg?
  2)  using the number of moles of Mg, how many moles of MgO will you produce?  (remember the balanced equation?)
  3)  using the number of moles of MgO, what is the mass of that many moles MgO?

There are other complications that can crop up, such as needing to convert to milligrams, etc., or using density to convert between mass and volume, but you still have these three basic steps.

Problems like #13, while perhaps common as exam questions, are quite trivial and not very common in real life (or even in a teaching lab), since we can't measure moles directly and they don't give you any directly useful information.  However, it is a very important calculation to know, since it is the calculation in step #2 above.

Hope this helps,
Dick

[KavX (OP)]

thanx guys, made the question seem simpler,