Chemistry - stoichiometry. Please please help!! i'm going crazy?

Question

i'm so damn stuck..

1. Fe(s) + CuSO4(aq) --> Cu(s) + FeSO4(aq)
What mass of copper would be obtained if 1.0 kg of scrap iron was added to enough copper sulfate to react all the iron?

2. What mass of iron is produced by reacting 25g of zinc with excess iron (III) oxide according to the equation:
Fe2O3(s) + 3Zn (s) --> 3ZnO + 2Fe(s)

3. Hydrogen burns in oxygen to produce water.
(a) write a balanced equation for this reaction
(b) calculate the mass of oxygen required to react with 10.0 g
of hydrogen.


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Thanks heaps!! and just do what u could be bothered doing. much appreciated!! so are stars!

thanks so much!! all these answers have helped heaps!
just regarding question 2 though.

I thought u did the mol times the ratio.. wouldnt it be this:
0.3846 * (3/2) = 0.5769

i think i get it.

its: amount of required substance / amount of given substance
so the ratio would be (2/3)

Answer 1

For the first question,
we see that the mole ratio for Fe : CuSO4: FeSO4 is 1:1:1
This means that you need 1 mole of Fe to react with 1 mole of CuSO4 and you will get 1 mole of FeSO4.

Since we have 1.0 kg of scrap iron, thats 1000g.
To get the number of moles, we divide it by the molar mass -->
1000g/56g = 17.86 mole (4sf)
Therefore we will get 17.86 mole of Cu, whose molar mass is 64. So 17.86 x 64 = 1143.04g = 1.143kg of Cu.

Q2:

Moles of Zn --> 25g/molar mass -->
25g/65g = 0.3846 (4sf)
We see that the mole ratio of Zn : Fe is 3 : 2
So, Zn : Fe is
0.3846 : (0.3846 / 3) x 2
Which is
0.3846 : 0.2564

So the mass of iron is 0.2564 moles x molar mass
Which is 0.2564 x 56 = 14.4g (3sf)

Q3:

a) 2H2(g) + O2(g) --> 2H2O(l)

b) Same as above.
Mole ratio of Hydrogen : Oxygen is 2 : 1
So, we find the moles of hydrogen.

10.0g/molar mass --> 10g/1+1 = 5 moles.
The moles of Oxygen is thus 5/2 = 2.5 moles.
The mass is 2.5 moles x molar mass, which is -->
2.5 x (16+16) = 80.0g (3sf)

Hope that helped, i just finished the chapter of mole concept last month.

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!!!

Nono, see if we used another example, A : B is 2 : 1.
If A is 20g, to find B,
you wouldn't use (2/1)(20) = 40 would you?
You'd use (1/2)(20) = 10.

Similarly, you are trying to find Fe.
Fe is 2/3 the times of Zn, so you x (2/3).
Zinc is the one that is 3/2 the times of Fe.
I think you got a little mixed up.

Answer 2

Each one of these problems is a mass to mass problem. There is no direct way to go from the mass of one substance to the mass of another. One must go through moles first, which relates number of particles that are valable to collide for the reaction in the ratios necessary.

I'll address each problem individually to show the pattern in each, as the way to work them is the same for each.

1. Mass given: 1 kg of Fe
Mass wanted: g of Cu

Things needed to solve problem:
Molar Mass of Fe = 55.845 g/mol
Molar Mass of Cu = 63.546 g/mol
1.0 kg = 1000 g
Mole ratio Fe : Cu = 1:1

Pathway necessary to solve problem:
kg of Fe --> g of Fe
g of Fe --> mol Fe
mol Fe --> mol Cu
mol Cu --> g Cu

This is a four step problem
1 kg Fe = 1000 g Fe
1000 g Fe = 18 mol Fe (1000 / 55.845) I rounded to 2 significant figures since 1.0 only has 2 significant figures
18 mol Fe = 18 mol Cu (1:1 ratio)
18 mol Cu = 1143 g Cu (18 x 63.546)

Rounding the answer to only 2 sig figs, more than likely they are looking for 1100 g or 1.1 kg as your final answer. If sig figs are ignored, 1143 g is probably close enough to get credit.

2. Given: 25g Zn
Wanted: g Fe

Needed to do problem:
Molar Mass Zn = 65.409 g/mol
Molar Mass Fe = 55.845 g/mol
Mole ratio Zn : Fe = 3 : 2 (gotten from coefficient in front of Zn and Fe in the balanced chemical equation)

Working the problem:
25 g Zn = 0.38 mol Zn (25 / 65.409)
0.38 mol Zn = 0.25 mol Fe (3 : 2 ratio)
0.25 mol Fe = 14 g Fe

3. a. Easy enough. Remember that hydrogen and oxygen are diatomic molecules.
2 H2 + O2 --> 2 H2O (please remember numbers in chemical formulas are subscripts)

b. Given: 10.0 g H2
Wanted: g O2

needed information:
Molar Mass H2 = 2 g / mol
Molar Mass O2 = 32 g / mol
Mole Ratio H2 : O2 = 2 : 1

Work:
10.0 g H2 = 5 mol H2 (10 / 2)
5 mol H2 = 2.5 mol O2 (2 : 1 ratio)
2.5 mol O2 = 80 g O2

Your final answer, if they are picky about sig figs, should be 80.0 g O2

Hope this helps

Answer 3

You need to work out lots of homework problems. Sorry it is just rote repeatability of problems to get it down. I think a good work book with problems and solutions is the best way of getting it. Get a book like a Schaum's outline of chemistry, which has a chapter on Stoichiometry along with plenty of word problems and examples on how to solve it. If chemistry is critical to your career path, take this extra step and learn it...it is better to work now and get it , then not get it and work later and much much longer at Burger King

Answer 4

1. no of moles of iron = mass/molar mass
= 1000g/56g/mol
= 17.86 mol.
since, iron is the limiting reactant, mass of copper formed
= (17.86*64)g
= 1143g
= 1.143kg

2. no of moles of zinc = mass/molar mass
= 25g/65g/mol
= 0.3856 mol.
since, zinc is the limiting reactant,
no. of moles of iron produced = 0.3856/3*2 = 0.256 mol.
mass of iron produced = 0.256*56 = 14.4g
(the coefficients of the reactants and products in any chemical reaction gives you the relative ratio of moles, that's why i divided by 3 and then multiply by 2. Simple ratio!)

3.
a) 2H2(g) + O2(g) ----> 2H2O(l)

no. of moles of hydrogen = 10/2
= 5 mol.
molar mass of hydrogen is two because it exists as a diatomic molecule.
no. of moles of oxygen = 5/2 = 2.5 mol.
mass of oxygen = 2.5*(16) = 40g