Amount of Substance

Question 1

What is a mole?

Answer 1

It is a unit of measurement used in chemistry.
A mole is just a really large number of particles (specifically Avogadro’s number)

Question 2

What is Avogadro’s constant?

Answer 2

It is the exact number of particles in the mole. 6.022 x10^23
(doesn’t matter what the particle is)

Question 3

How can you convert between the number of particles and the number of moles?

Answer 3

Number of particles= number of moles x Avogadro’s constant

Question 4

How can you calculate moles?

Answer 4

moles= mass/mr

Question 5

What is the concentration of a solution?

Answer 5

The concentration of a solution is how many moles are dissolved per 1dm3 of solution. The units are mol dm^-3

Question 6

How do you find the number of moles in a solution?

Answer 6

Moles= concentration x volume

Question 7

How many cm3 are in a dm3

Answer 7

1000cm3 is 1dm3

Question 8

What is the ideal gas equation?

Answer 8

pV = nRT
p= pressure
V= volume
n= number of moles
R= gas constant
T= temperature

Question 9

How do you know if an equation is balanced?

Answer 9

There are equal numbers of each atom on both sides of the equation

Question 10

How do you write an ionic equation?

Answer 10

In an ionic equation, only the reacting particles and the products they form are included. cross out any ions present on both sides of the equation (spectator ions) and you get an ionic equation

Question 11

What are the four state symbols used in equations?

Answer 11

s= solid
l= liquid
g= gas
aq= aqueous

Question 12

How do you make a standard solution?

Answer 12

For example, making 250cm3 of a 2.00 mol dm3 of NaOH
1) First work out how many moles of NaOH using the concentration equation
2) Now work out how many grams of NaOH you need using the Mr formula
3) place a weighing boat on a digital balance and weigh out this mass of solid. then reweigh after removing the solid and calculate the precise mass of the solid used
4) Add distilled water to the beaker and stir until the NaOH is dissolved
5) Tip the solution into a volumetric flask using a funnel to make sure it’s all going in
6) Rinse the beaker and stirring rod with distilled water and add it to the flask too
7) Then top the flask up to the correct volume with distilled water. Making sure not to pass the line
8) Stopper the flask and turn it upside down a few times to make sure it is mixed

Question 13

Why are titrations useful and how do you set them up?

Answer 13

A titration tells you how much acid is needed to neutralize a measured quantity of alkali. You can use this data to work out the concentration of the alkali
1) Use a pipette to measure out a set volume of the solution that you want to find out the concentration and put it in a flask
2) Add a few drops of an indicator (e.g phenolphthalein)
3) Then fill a burette with a standard solution of the acid (known concentration)
4) Use a funnel to fill the burette and do it below eye level

Question 14

How do you carry out a titration?

Answer 14

1) First do a rough titration to get an idea of where the endpoint is. This is done by adding the acid using the burette slowly, giving the flask a regular swirl
2) Now do an accurate titration by running the acid to 2cm3 of the before endpoint and adding drop by drop until the indicator changes color
3) Work out the amount of acid used. Final reading- initial reading. This volume is known as the titre
4) Repeat the titration a few times until you have concordant results, then calculate an average.

Question 15

Why are methyl orange and phenolphthalein good indicators for titrations, and what colors do they go?

Answer 15

They change color quickly over a small pH range so you can see exactly when the reaction has ended.
Methyl orange is red in acid and yellow in alkali
Phenolphthalein is colorless in acid and pink in alkali
(good idea to view the color change on a white tile)

Question 16

What is an empirical formula?

Answer 16

The empirical formula gives the simplest whole-number ratio of atoms of each element in a compound.

Question 17

What is the molecular formula?

Answer 17

The molecular formula gives the actual number of atoms of each element in a compound. The molecular formula is made up of a whole number of empirical units.

Question 18

What is the theoretic yield?

Answer 18

The theoretical yield is the mass of the product that should be formed in a chemical reaction (it assumes no chemicals are lost in the reaction). You can use the masses of reactants and a balanced equation to calculate the theoretical yield for a reaction.

Question 19

For any reaction why is the actual yield sometimes less than the theoretical yield?

Answer 19

• not all reactants have reacted fully
• impurities in the reactants
• some chemicals are lost (e.g. left on filter paper)
• ## the reaction is reversible

Question 20

What is the equation for percentage yield?

Answer 20

Percentage yield= actual yield/theoretical yield x100

Question 21

What information does percentage yield give us?

Answer 21

It tells us how wasteful a process is/efficient

Question 22

What is atom economy?

Answer 22

Atom economy is a measure of the proportion of reactant atoms that become part of the desired product in the balanced chemical equation

Question 23

What is the equation for atom economy?

Answer 23

% atom economy = molecular mass of desired product/ sum of molecular masses of all reactants
x100

Question 24

Why do companies in the chemical industry prefer processes with high atom economies?

Answer 24

• They are better for the environment because they produce less waste. Waste needs to be disposed of safely so the less that is made- the better.
• They make more efficient use of raw materials, so they’re more sustainable
• They are also less expensive as they spend less on separating the desired product from the waste products and also less on treating waste

Question 25

What is the Density equation?

Answer 25

Density= mass/volume

Question 26

In a titration why is the volumetric flask containing your standard solution inverted multiple times?

Answer 26

So that the concentration is evenly distributed

Question 27

If you have already rinsed the burette and pipette with water why do you need to rinse them with solution?

Answer 27

The water could stick to the glassware and dilute the solution that is being added so rinsing with the solution will not change the concentration.

Question 28

Why should you not force the solution out of the pipette with the filler if it is faster?

Answer 28

The filler leaves a small volume of solution at the bottom of the pipette which is accounted for which will occur due to capillary action from gravity. It could also damage the pipette.

Question 29

Given that the uncertainty of a titration came mainly from the burette how could you reduce the percentage uncertainty?

Answer 29

Use a more diluted acid

Question 30

What are the potential errors when using a gas syringe?

Answer 30

Gas escapes before the bung is inserted
Syringe sticks

Question 31

How to decrease the percentage uncertainty when measuring masses?

Answer 31

Weigh before and after to find mass transferred