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Chemistry Problems

1.) Elemental phosphorus reacts with chlorine gas according to the equation:
P4(s)+6Cl2(g)→4PCl3(l)
A reaction mixture initially contains 45.09g P4 and 131.1g Cl2.
 
Once the reaction has occurred as completely as possible, what mass (in g) of the excess reactant is left?
 
 
 
2.) Iron(II) sulfide reacts with hydrochloric acid according to the reaction:
FeS(s)+2HCl(aq)→FeCl2(s)+H2S(g)
A reaction mixture initially contains 0.214mol FeS and 0.664mol HCl.
 
Once the reaction has occurred as completely as possible, what amount (in moles) of the excess reactant is left?
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3 Answers

The chemical equation when balanced shows the number of moles of each chemical by the coefficients.
This equation shows 1 mole P4 reacts with 6 moles Cl2.
The molar masses for each reactant are (4x30.97) or 123.88g for P4 and (2x35.45) or 70.90g for Cl2.
45.09 g of P4 divided by 123.88g/mol = .3640 mol of P4.
131.1g of Cl2 divided by 70.90g/mol = 1.849 mol of Cl2.
Since there is a 1:6 ratio of P4 to Cl2, divide 1.849 mol by 6 to get .3082 mol Cl2 compared to .3640 mol P4.
The difference is .0558 mol of excess P4.
Multiply by 123.88g/mol of P4 to get 6.913g of excess P4.
This answer has 4 significant digits like the given data.
Question 1
 
45.09g of P4 ÷ 123.896 g/mol = .363 mol P4
 
131.1g of Cl2 ÷ 70.91 g/mol = 1.849 mol Cl2
 
The balanced equation shows you need 1 mole of P4 for every 6 moles of Cl2
 
If you divide moles of Cl2 by 6, you will get the number of moles of P4 required.
 
1.849 / 6 = .308 moles of P4
 
Given we have .363 moles of P4, we conclude P4 is in excess.
 
The amount left over is .363 - .308 = .055 mol = 6.81 g
 
You can do the second question following my example.
 
I will do the second question because there is a more direct and sensible way to think about and approach the question than offered by others.
 
This question is easier than the first because the quantities are given in moles rather than grams so we don't even need to do the conversion to moles.
 
So we are given 0.214 mol FeS and 0.664 mol HCl.
From the balanced equation we know for every 1 mole of FeS we require 2 moles of HCl. So for .214 mol FeS we require .214 x 2 = .428 mol HCl. The amount we have is .664 mol which is more than enough making HCl the excess reagent
 
Therefore .664 - .428 = .236 mol HCl left over.
1) With our reaction formula we have to determine the limiting reactant. To do that you have to convert the initial reactants from grams to moles.
 
1 mol of P4 = 123.88 g so....
45.09g of P4 / 123.88 g = 0.364 moles of P4
 
1 mol of Cl2 = 70.9 g so.....
131.1 g of Cl2 / 70.9 = 1.8491 moles of Cl2
 
The next step is to look at the coefficients of the rxn formula P4(s) results in 4PCl3(l). Since the reactsant and the product both have different coefficient this is a 1:4 ratio. 0.364 moles of P4 will result in 1.456 moles of PCl3(l) if it is the limiting reactant.
 
If we look that the other reactant 6Cl2(g) results in 4PCl3(l). The reactant and product have different coefficient so determining how much 4PCl3(l) is produced from 1.8491 moles of Cl2 isn't as simple. To do this we step up a proportion equation
 
4PCl3(l) / 6Cl2(g) = x / 1.8491 moles of Cl2
4 / 6 = x / 1.8491
(4 / 6) * 1.8491 = x
x = 1.2327 moles of PCl3(l)
 
Since Cl2(g) produces less product than P4 (s), that means that  Cl2(g) is the limiting reactant and that there will be an excess of P4 (s). Determining how much is left over is easy. Since  the Cl2(g) only produced  1.2327 moles of PCl3(l) we subtact that from the amou t that P4 (s) would have produced if it was the limiting reactant.
 
1.456 moles of PCl3(l) - 1.2327 moles of PCl3(l) =
1.456 - 1.2327 =
0.2233 moles of PCl3(l)
 
and since P4 (s) and PCl3(l) have a 1:4 ratio, this answer must be divided by 4 which means 0.2233 moles of PCl3(l) = 0.0.055825 moles of P4 (s). From here all that is left is to convert moles of P4 (s) to grams of P4 (s).
 
1 mol of P4(s) = 123.88 g so....
0.055825 moles of P4 (s) x 123.88 g =
6.916 g of P (s) in excess <======= ANSWER
 
2) This problem is solved in a similar manner so I will skip a lot of the explanations
 
FeS(s)+2HCl(aq)→FeCl2(s)+H2S(g)
 
FeS(s) => FeCl2(s) (1:1 ratio)
0.214 mol FeS = 0.214 mol FeCl2          
 
2HCl(aq) => FeCl2(s) (2:1 ratio)
0.664 mol HCl =  0.332 mol  FeCl2
 
FeS is limiting reactant so......
 
0.332 mol FeCl2 - 0.214 mol FeCl2 = 0.118 mol FeCl2
 
2HCl(aq) => FeCl2(s) (2:1 ratio)
 
2HCl(aq) / FeCl2(s) = x mol HCl / 0.118 mol FeCl2
2/1 = x/ 0.118
2 * 0.118 = x
x = 0.236 mol of excess HCl <======= ANSWER
 

Comments

this reaction requires P in gaseous state so P4 is correct, the (s) should be (g)
ty....noticed after...lol
White phosphorus is a tetrahedral structure with 4 P atoms, therefor P4 is suitable as the formula for solid phosphorus. (See Wikipedia on "Phosphorus")