Amount of Substance - Calculating Percentage Yields (A-Level Chemistry)
Calculating Percentage Yields
Percentage Yield
Theoretical Yield
The theoretical yield is the quantity of the product that should be obtained in the reaction if it goes to completion. It is the maximum product you could get. An assumption is made when calculating the theoretical yield that there is no transfer of chemicals to or from the reaction. In other words, that neither reactants nor products are lost in the reaction.
Actual Yield
The actual yield is the amount of product obtained in reality in the reaction. The actual yield is always smaller than the theoretical yield because:
- Some of the product could be lost. Some product can be lost – e.g. when the reaction mixture is transferred between containers or when a solution is filtered using filter paper.
- Some reactants don’t react. Not all of the reacting chemicals at the start of the reaction will completely react. These reacting chemicals may remain unreacted until the reaction is finished.
- Other waste products could be made. Some of the reactants in the reaction may go on to form other products rather than the desired products. This will occur if there are side reactions that are happening at the same time as the desired reaction. If you wish to make magnesium hydroxide by reacting magnesium with water (Mg + 2H2O → Mg(OH)2 + H2), then any hydrogen gas produced is a waste product.
Percentage Yield
The percentage yield is the the relationship between the actual yield and the theoretical yield. It can be defined by this equation:
Calculations involving Percentage Yield
You need to be able to carry out calculations involving percentage yield in your exams.
In exam questions you are often provided with the actual yield and then asked to calculate the theoretical yield. Once you know the values for both the actual yield and the theoretical yield, you can calculate the percentage yield using the equation above.
Practice Question: Tom is carrying out a reaction between methanol (CH3OH) and excess ethanoic acid (CH3COOH) in the lab. He reacts 5g of methanol with excess acid. He measures the mass of methyl ethanoate (CH3OOCCH3) produced, and finds that it is 9.6g. What is the percentage yield of this reaction? (Ar values are as follows, C = 12, H = 1, O = 16)
1. First of all write out a balanced equation for the reaction.
CH3OH + CH3COOH —> CH3OOCCH3
2. Next write down the relative relative molecular masses (Mr) of the substances mentioned in the question.
Mr of CH3OH = 12 + (1 x 4) + 16 = 32
Mr of CH3COOH = (12 x 2) + (1 x 4) + (16 x 2) = 60
Mr of CH3OOCCH3 = (12 x 3) + (1 x 6) + (16 x 2) = 74
3. Then calculate the number of moles of methanol (CH3OH) you started with.
Number of moles of CH3OH that reacts = 5g ÷ 32 = 0.15625 moles
4. Now calculate how many moles of methyl methanoate are formed.
According to the equation, the ratio of the number of moles of Methanol to Methyl Methanoate in the reaction is 1:1
This means when 0.15625 moles of methanol reacts, 0.15625 moles of methyl methanoate is formed.
5. Calculate the theoretical yield (g) of methyl methanoate.
Theoretical yield of (CH3OOCCH3) = number of moles x Mr = 0.15625 x 74 = 11.6g
6. Finally calculate the percentage yield
Calculate the percentage yield by using the values of the actual yield that is provided in the question and the theoretical yield that you calculated in step 5.
Percentage yield = (actual yield ÷ theoretical yield) x 100= (9.6 ÷ 11.6) x 100= 82.8%
The amount of substance, also known as the chemical amount or amount of matter, refers to the amount of a substance that is present in a sample. It is usually measured in units of moles.
Calculating the amount of substance in a chemical reaction is important because it helps to determine the amount of each reactant and product involved in the reaction. This information is crucial for determining the stoichiometry of the reaction, which is the ratio of the amount of reactants to products.
A mole is the unit of measurement used to express the amount of substance in a sample of a chemical substance. One mole of a substance contains Avogadro’s number of atoms, ions, or molecules of that substance.
Percentage yield is a measure of the efficiency of a chemical reaction, calculated as the ratio of the actual yield of a product to the theoretical yield of the product, multiplied by 100. The theoretical yield is calculated based on the stoichiometry of the reaction, while the actual yield is determined by weighing the product after the reaction has taken place.
Percentage yield is calculated using the following formula:
Percentage yield = (actual yield / theoretical yield) x 100
The percentage yield of a chemical reaction can be affected by a number of factors, including the purity of the reactants, the temperature and pressure conditions, the presence of catalysts or inhibitors, and the rate at which the reaction takes place.
A low percentage yield in a chemical reaction indicates that the reaction is not very efficient, and that some of the reactants are not being fully converted into products. This can be due to a number of factors, including the presence of impurities, reaction conditions that are not ideal, or the presence of side reactions that consume some of the reactants.
Having a high percentage yield in a chemical reaction is important because it indicates that the reaction is efficient and that the maximum amount of product is being produced. This can help to increase the yield of a product and reduce the cost of producing chemicals in industry.
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