Limiting Reagent Help?
how do you use mass/mass conversions to determine the limiting reagent given a balanced equation. also, how do you use limiting reagent and actual yield to determine percent yield?
please help! my final is tomorrow and i'm so confused!
Answer:
If you start with grams of both the reactants, you should convert both of them all the way to grams of the product. The steps should be:
(g reactant)(1mol/molar mass of reactant)(coefficient of product/coefficient of reactant)(molar mass of product/1 mol)
If you do this for both the reactants, the limiting one is the one that produces the smaller amount of product. That's because the limiting reagent is the one that is completely used up and so you can only make as much product as it will allow.
Think of it like trying to make a sandwich. If you are making cheese sandwiches, with one piece of cheese on two pieces of bread, the number of sandwiches you can make depends on your ingredients. Let's say you have ten pieces of bread and 8 pieces of cheese. You can only make 5 total sandwiches because of the bread and you will have left over cheese (3 pieces). So the limiting reagent is the bread.
Once you determine which one is the limiting one, that amount of product you calculated is the theoretical yield. This is how much you should produce if every bit of the limiting reagent is used up. (Like in our sandwich example, the theoreticla yield is 5 sandwiches).
The formula for percent yield is"
% yield = actual yield / theoretical yield
so you take the actual yield (which is given to you in the problem as grams of a product or data from a lab) and divide it by the calculated theoretical yield. Then put it in terms of a percentage by multipling it by 100.
The limiting reagent is the element or compound that is used up FIRST in a reaction. Generally it is used up before any other causing the reaction to cease. It LIMITS the reaction by being consumed first.
I'll use the raction of Zinc in HCl as an example.
Zinc metal + Hydrochloric acid -> Zinc Chloride and Hydrogen Gas
Zn + 2HCl --> ZnCl2 + H2
Now let's that that you reacted 130g of Zn with an infinite amount of HCl and you want to figure out how much HCl you will need. On chem test they say "excess" instead of infinite, but both just mean that all of your reagent will react since there are no other limiting reagents.
130g of Zinc is 2 moles of Zinc.
130g Zn * (1 mol / 65g/mol Zn) = 2 mol Zn
Now the trick y part. You need to decide how much HCl you need in relation to your zinc. So, take a look at the number of moles in the equation for the chemicals you're reacting: Zn + 2HCl. 1 mol of zinc, 2 moles of HCl.
Multiply the number of moles of zinc you have by the mole ratio (2 mol HCl / 1 mol Zn) and you'll get the number of moles of HCl needed.
2 mol Zn * (2 mol HCl / 1 mol Zn) = 4 mol HCl
To figure out which reagent is the limiting it is the same process, you just do it twice.
Using the same reaction (Zn + 2HCl --> ZnCl2 + H2) we'll figure out if 130g of Zinc is limiting when producing zinc chloride (ZNCl2), or if 50g of HCl is.
You already know that 130g of Zing is equal to 2 mols, so skip to multiplying by the mole ratio.
2 mol Zn * (1 mol ZnCl2 / 1 mol Zn) = 2 mol ZnCl2
2 mols of zinc react to yeild 2 mol ZnCl2
Now for the HCl:
50g HCl * (1mol / 36.4g/mol HCl) = 1.37mol HCl
1.37mol HCl * (1 mol ZnCl2 / 2 mol HCl) = .685 mol ZnCl2
The amount of HCl you have present will only allow you to produce .685 mols of product, so it is limiting.
The key to realize when solving limiting reactant problems is that it isn't all that important how the masses of the two substances compare, but their moles. The first step is to calculate the molar masses of the substances you have to determine the number of moles you have;
_g/molar mass= # of moles
Now that you have moles, you need to examine your chemical equation.
Say you have:
1NaCl + 1HOH -> 1NaOH + 1HCl
Probably not the best example realistically, but it works.
Anyway, you need to examine the coefficients infront of each product. Since there is all 1's, then for every mol of NaCl, 1 mol of HOH is needed. If you have more moles of NaCl, for example, then HOH is the limiting reactant.
Good luck.
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