Calculating Yield

L.O: To be able to calculate yield for chemical reactions

What does yield mean?

The amount of product made is called the yield.In a chemical reaction no atoms are lost or

gained but sometimes the yield is not what you would expect.

Theoretical yield: maximum products that are made if reactants react.

Actual yield: the amount of product which actually forms

Can you predict exactly the mass of a yield?

Sodium hydroxide + hydrochloric acid sodium chloride + hydrogen

If 10g of NaOH was used how much salt would be

made?

What if a company wants to make 100g sodium

chloride, how much NaOH should they use?

• Precipitation is the formation of an insoluble solid when two solutions are mixed - e.g. barium sulphate is produced by precipitation from barium nitrate and sodium sulphate solutions

Write a word equation for this reactionSodium Sulphate +Barium Nitrate Sodium Nitrate +Barium Sulphate

Na2SO4 + Ba(NO3)2 → 2NaNO3 + BaSO4

1. 50cm3 water into a 100cm3 beaker2. Weigh 2.6g barium nitrate3. Combine the two and stir (until all barium nitrate is

dissolved)4. Pour this into the 250cm3 beaker5. Measure out 75cm3 sodium sulphate into a 100cm3 beaker6. Add the two solutions together7. Stir well (notice the white precipitate)8. Filter the mixture using a funnel and filter paper - wash the

residue with a little water9. Can you calculate the theoretical yield of barium sulfate?

Then next lesson we can compare this to the actual yield produced

Barium Sulphate precipitationBarium Sulphate precipitation

Calculating the theoretical yield

Step 1: Calculate the formula mass of the reactantsHere are the mass numbers you will need: Barium =

137 Nitrogen = 14 Oxygen = 16

What is the formula mass of barium nitrate Ba(NO3)2 ?Barium = 137 x 1 = 137Nitrogen = 14 x 2 = 28Oxygen = (16 x 3) x 2 = 96Total = 261

Step 2: Calculate the relative formula mass of barium sulphate

(the product formed):BaSO4

Barium = 137 Sulphur = 32 Oxygen = 16

And the relative formula mass is: -Barium = 137 x 1 = 137Sulphur = 32 x 1 = 32Oxygen = 16 x 4 = 64Total = 233

Na2SO4 + Ba(NO3)2 → 2NaNO3 + BaSO4 + 261g → + 233g

• This means that 261g of barium nitrate will produce a theoretical 100% yield 233g of barium sulphate.

Step 3: (Scaling up or down the reaction) Now work out what the mass is of the product made from 1g of the reactant. Then multiply this by the mass of the reactant that was used

So 1g of Ba(NO3)2 makes 233/261 = 0.9g of BaSO4

2.6g of Ba(NO3)2 makes 0.9 x 2.6= 2.3g of BaSO4

Calculating expected yield1. In the following reaction if N2(g) is present in

excess and 12 g of H2(g) reacts, what is the theoretical yield of NH3(g)?

3 H2(g) + N2(g) 2 NH3(g)

2. If 11.5g of sodium is reacted with chlorine what is the theoretical yield of sodium chloride

2Na(s) + Cl2(g) 2NaCl(s)

Some more practice – Calculating Mass in reactions

Calculating the mass of a productE.g. what mass of magnesium oxide is produced when 60g of magnesium

is burned in air?

Step 1: READ the equation:

2Mg + O2 2MgO

IGNORE the oxygen in step 2 – the question

doesn’t ask for it

Step 3: LEARN and APPLY the following 3 points:

1) 48g of Mg makes 80g of MgO

2) 1g of Mg makes 80/48 = 1.66g of MgO

3) 60g of Mg makes 1.66 x 60 = 100g of MgO

Step 2: WORK OUT the relative formula masses (Mr):

2Mg = 2 x 24 = 48 2MgO = 2 x (24+16) = 80

Work our what mass 1g makes then work out

what mass 60g of mg makes

(Another method)Calculating the mass of a productE.g. what mass of magnesium oxide is produced when 60g of magnesium

is burned in air?

Step 1: READ the equation:

2Mg + O2 2MgO

IGNORE the oxygen in step 2 – the question

doesn’t ask for it

Step 3: work out how many moles reacted:

1) Moles = mass ÷ formula mass. 60g / 48g = 1.25 moles

2) Moles X formula mass of product = 1.25 X 80 = 100g

3) 60g of Mg makes 100g of MgO

Step 2: WORK OUT the relative formula masses (Mr):

Mg = 24 2MgO = 2 x (24+16) = 80

Work out the mass of 2.5 moles of the

product (same

number of moles as the

reactants)

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Work out Mr: 2H2O = 2 x ((2x1)+16) = 36 2H2 = 2x2 = 4

1. 36g of water produces 4g of hydrogen

2. So 1g of water produces 4/36 = 0.11g of hydrogen

3. 6g of water will produce (4/36) x 6 = 0.66g of hydrogen

Mr: 2Ca = 2x40 = 80 2CaO = 2 x (40+16) = 112

80g produces 112g so 10g produces (112/80) x 10 = 14g of CaO

Mr: 2Al2O3 = 2x((2x27)+(3x16)) = 204 4Al = 4x27 = 108

204g produces 108g so 100g produces (108/204) x 100 = 52.9g of Al2O3

1) When water is electrolysed it breaks down into hydrogen and oxygen:

2H2O 2H2 + O2

What mass of hydrogen is produced by the electrolysis of 6g of water?

3) What mass of aluminium is produced from 100g of aluminium oxide?

2Al2O3 4Al + 3O2

2) What mass of calcium oxide is produced when 10g of calcium burns?

2Ca + O2 2CaO

A high yield is important in business. Can you list any ways that business could ensure a high yield?

e.g. when making nylon, plastic, perfume or margarine

• Give it enough time to finish• Try not to loose any reactants• Ensure the reactants are pure• Makesure the conditions are best e.g.

temperature

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Conclusion – why isn’t it possible to collect the theorectical yield?

1) The reaction may not have completely _______

2) The reaction may have been _______

3) Some of the product may have been ____

4) Some of the reactants may have produced other _______

The amount of product that is made is called the “____”. This number can be compared to the maximum theoretical amount as a percentage, called the “percentage yield”.

Words – lost, yield, finished, reversible, products