C5

Question 1

How can you calculate theoretical yield?

Answer 1

Theoretical yield =
[mass of limiting reactant/ sum of Mr for limiting reactant ] x sum of Mr of products

Question 2

What is the theoretical yield

Answer 2

The maximum mass it is possible to make from a given mass of reactants.

Question 3

How do you calculate percentage yield?

Answer 3

[actual yield / theoretical yield] x 100

Question 4

What is the actual yield?

Answer 4

The mass of the product you actually make in a chemical reaction. (Usually less than theoretical yield)

Question 5

What affects percentage yield?

Answer 5

-Reaction may not go to completion
-The reactants may react in a different way than expected
-you may lose some of the product during a purifying process

Question 6

How do you calculate the atom economy?

Answer 6

Atom economy = [sum of Mr of the desired product/ sum of Mr of all products] x 100

Question 7

What factors determine how you make a substance?

Answer 7

-Yield of the product
-Atom economy of the reaction
-Usefulness or otherwise of by-products
-rate of the reaction
-equilibrium position, if it is a reversible reaction.

Question 8

What is a by-product

Answer 8

A by-product is a substance formed in a reaction in addition to the desired product

Question 9

How do you convert centimetres cubed to decimetres cubed?

Answer 9

Divide by 1000

Question 10

How do you convert decimetres cubed to centimetres cubed?

Answer 10

Multiply by 1000

Question 11

How do you calculate concentrations?

Answer 11

Concentration in g/dm3 = [mass of solute in g/ volume of solution in dm3]

Question 12

How do you calculate concentration in moles per cubic decimetre, mol/dm3

Answer 12

Concentration in mol/dm3 =
[amount of solute in mol/volume of solution in dm3]

Question 13

What is a titration?

Answer 13

In a titration you usually add a known volume of alkali solution to a conical flask, and a few drops of a single indicator.
You then add the acid to the flask with a burette, a king graduated glass tube with a tap bottom.
At the end point when the alkali is neutralised and the indicator first changes colour you stop adding acid.

Question 14

How do you do a titration

Answer 14

When doing a titration you need to take an initial burette reading and a final burette reading at the end-point. The difference between these two readings is the titre, the volume of acid added to the alkali in the flask. Record your readings to two decimal places, ending in 0 if the bottom of the meniscus is on a burette line, or 5 if the meniscus is between two lines.
To obtain a repeatable titre you need to use the following techniques.

1-Swirl the flask during a titration to mix its contents.

2 -Your first titration is usually a rough run, done quickly so that you get an idea of what the titre is.

In later runs, you can quickly add the acid to within a few cm° of the rough titre, then add the acid drop by drop.

Repeat the titration until you obtain at least two concordant titres, titres that are within 0.10 cm? of each other.

This allows you to calculate a mean titre that has high precision.

Eye protection should be worn throughout this practical.

Question 15

How can you use results from a titration?

Answer 15

When you have carried out a titration, you will know:

-The two reactants used (an acid and an alkali)
-The volume and concentration of one of the reactants
-The volume, but not the concentration of the other reactant.
(Use equation to calculate the other reactant’s concentration)

Question 16

What is the molar volume?

Answer 16

One mole of any volume I the gas state occupies the same volume at the same temperature and pressure. This volume is the molar volume,Vm.

Question 17

What is the molar volume at room temperature?

Answer 17

Vm= 24dm3/mol (24000cm3/mol)

Question 18

This equation links the amount of volume of a substance in the gas state at room temperature:

Answer 18

Volume in dm3 = amount in mol x 24dm3/mol

Question 19

Measuring the volume of a gas

Answer 19

You can measure the volume of gas produced using a measuring cylinder.
1 -Fill the measuring cylinder with water and, while keeping its mouth underwater, turn it upside down.
2 -Clamp the measuring cylinder securely (Figure 3).
3 -Mix the reactants in a conical flask and attach a delivery tube.
4 -Measure the volume of gas produced in the reaction by recording the end reading on the measuring cylinder.
In an this investigation, you need to keep the mouth of the measuring cylinder underwater (Figure 3). This is in order to stop air getting in or water getting out. Either of these would affect the volume reading on the measuring cylinder.
Eye protection should be worn throughout this practical.

Question 20

How do you use the molar volume?

Answer 20

-you can use the molar volume to calculate:

The volume of a gaseous reactant or product, if you know it’s amount.
The amount of a gaseous reactant or product, if you know it’s volume.

Question 21

What are the two ways to calculate the rate of reaction?

Answer 21

Rate of reaction = [amount of reactant used/time taken]

Rate of reaction = [amount of product formed/time taken]

Question 22

How do you measure the volume of a gas?

Answer 22

You have already looked at measuring the volume of gas produced in a reaction using an upside-down measuring cylinder in water. Another method is to use a gas syringe (Figure 2). A gas syringe is made from glass, and has graduations marked in cm?. As the syringe fills, the plunger moves outwards and you can record the volume of gas it contains. You are then able to calculate the rate of reaction.

Question 23

Measuring the volume of gas

Answer 23

Magnesium reacts with dilute hydrochloric acid to form magnesium chloride solution and hydrogen:
Mg(s) + 2HCI(aq) —>MgC1(aq) + H,(g)
You can measure how fast hydrogen is produced by carrying out the following steps.
1 Place dilute hydrochloric acid in a conical flask connected to a gas syringe.
2 Add a piece of magnesium ribbon into the acid, stopper the flask, and start a stop clock.
3 Draw a results table and record the time and volume of hydrogen at regular intervals.
4 Analyse your results by drawing a line graph as in Figure 3.

Before adding the stopper to the conical flask in this investigation, you should push the plunger all the way in, to make sure the reading starts at 0cm?.
You also need to make sure that you stopper the flask as soon as the reaction starts, in order to make sure that all the hydrogen is collected.