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chemistry igcse acapprinted 20 · PDF 20
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printed 20 · PDF 20
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20
= 0.5 : x
2x = 3.5
x = 1.75
• This means that to completely react with 0.5 mol. of , there needs to be 1.75 . However, as we calculated
previously, there is only 1.4063 present, which is less than the required amount to completely react. Thus,
we can deduce is the limiting reagent in this question, and therefore is the excess reagent. This means that
we must compare with to find the number of grams of water produced.
Step 3: Calculate the moles of water formed.
=
7
: 6
= 1.4063 : x
7x = 8.4378
x = 1.2054
Step 4: Calculate the mass of water.
Percentage Yield
• Yield is the term used to describe the amount of product you get from a reaction.
• In practice, you never get 100% yield in a chemical process for several reasons:
• Some reactants may be left behind in the equipment.
• The reaction may be reversible and in these reactions a high yield is never possible as the
products are continually turning back into the reactants.
• Some products may also be lost during separation and purification stages such as filtration
or distillation.
• There may be side reactions occurring where a substance reacts with a gas in the air or an
impurity in one of the reactants.
• Products can also be lost during transfer from one container to another.
Actual & Theoretical Yield