Original post by samb1234Right I'm going to go for a bit of a different approach, I'm going to do a fully worked example explaining each stage as I go to hopefully allow you to answer anything they ask. Note that I'm just making this question up it doesn't come from anywhere.
Question: Sodium metal reacts with chlorine gas to form a white crystalline soild of sodium chloride. John reacts 24.2g of sodium metal with 1 mole of chlorine gas, and he produces 26.4g of sodium chloride. Calculate his % yield.
Step 1: Write out the full balanced symbol equation, so firstly you need to work out the formula of the product. Sodium is a group one metal so will form a 1+ ion, and chlorine is in group 7 so will form a 1- ion. Therefore the formula of the product is NaCl. We know that chlorine is diatomic so the initial, unbalanced equation is:
Na +Cl2 -> NaCl
To balance we need there to be the same amount of each element on both sides of the equation . At the moment there are 2 chlorines on the left and only one on the right, so we need 2 NaCl:
Na +Cl2 -> 2NaCl
We now have the right number of chlorines on both sides, but we now have 2 sodiums on the right and only one on the left, so we need 2 Na so the final, balanced equation would be:
2Na +Cl2 -> 2NaCl
Step 2
Now that we have the equation, we first want to work out the number of moles we have of every reagent. In this example, we are told that we have one mole of chlorine. We have 24.2g of sodium metal, and one mole of sodium would weigh 23g. Therefore we have:
24.2/23 = 1.05... moles of sodium, and 1 mole of chlorine (given in question in this example, but if it wasn't you just work out the number of moles of all the reagents makes no difference).
Step 3
We now need to look at the balanced equation and the number of moles we have to work out the limiting reagent. What does this mean? Well imagine you were making a cake, and each cake took 3eggs and a kg of flour. If you only had 3kg of flour, even if you had 100 eggs you would still only be able to make 3 cakes, so the flour would be the limiting reagent.
So we know from the balanced equation that for every 2 moles of sodium reacting we need 1 mole of chlorine. However, you should clearly be able to see that we do not have twice as much sodium as there is chlorine. Even though we have 1 mole of chlorine, since we do not have 2 moles of sodium the chlorine is in excess.
Step 4
Now that we have identified the sodium as the limiting reagent, we are able to establish the expected yield. Looking back at the original equation, the number of moles of Na is the same as the number of moles of NaCl. Therefore if we have 1.05... moles of Na, we would theoretically expect to get 1.05... moles of NaCl.
Therefore the expected yield in grams would be (mass of one mole) x (number of moles we have). The expected yield is therefore 1.05... x (23+35.5) = 61.6g
Step 5
Now we have the expected yield we can work out the % yield.
% yield = mass we got/ expected mass x 100
= 26.4/61.6 *100 = 42.9%
Hope that helped