Topic 5- Formula, equations and amounts of substance

Question 1

Define empirical formula

Answer 1

shows the smallest whole-number ratio of each element in a compund

Question 2

Give an example of an experimental method to determine the formula of a black copper oxide

Answer 2

1. place a known mass of copper oxide in the tube
2. heat it in a stream of hydrogen gas
3. hydrogen atoms and oxygen atoms in the oxide react to from steam
4. the solid gradually changes colour from black to orange-brown
5. excess gas burned off at the end of the tube for safety reasons
6. after cooling, remove and weigh the solid copper
7. heat solid again in stream of hydrogen and check whether its mass changes (heat to a constant mass)

Question 3

How would you calculate the empirical formula using data from the experimental method?

Answer 3

1. divide by mass, percentage composition by mass, of each element by its relative atomic mass
2. divide the answers from this step by the smallest number to determine the ratio
3. whole numbers found in the ratio are used to write the empirical formula

Question 4

Give the equation to work out moles using mass and Mr

Answer 4

moles=mass(g)/ Mr (gmol-1)

Question 5

Define molecular formula

Answer 5

shows the actual number of atoms of each element in a molecule

Question 6

Define molar mass

Answer 6

the mass per mole of substance. Has the symbol M and units gmol-1

Question 7

What do each of these SI units mean: pV=nRT?

Answer 7

p=pressure in pascals, Pa
V=volume in m3
T=temperature in K
n=amount of substance in mols
R=the gas constant, 8.31Jmol-1K-1

Question 8

What’s a mole?

Answer 8

the amount of substance that contains the same number of particles as the number of carbon atoms in exactly 12g of the carbon-12 isotope

Question 9

Define Avogado’s constant

Answer 9

the number of atoms of C12 in exactly 12g of C12

Question 10

What are spectator ions?

Answer 10

ions in an ionic compound that do not take part in the reaction

Question 11

Define coefficient

Answer 11

number written in front of a species when balancing an equation

Question 12

What are hydrates?

Answer 12

compounds containing water of crystallisation

Question 13

What’s the Avogadro’s law?

Answer 13

states that equal volumes of gases under the same conditions of temperature and pressure contain the same numbers of molecules

Question 14

Define the molar volume

Answer 14

the volume occupied by 1 mol of any gas

Question 15

What’s a solute?

Answer 15

a substance that is dissolved

Question 16

What’s a solvent?

Answer 16

a substance that dissolves a solute

Question 17

What’s a solution?

Answer 17

a solute dissolved in a solute

Question 18

What’s the mass concentration of a solution?

Answer 18

the mass (in g) of the solute divided by the volume of the solution

Question 19

What’s the molar concentration of a solution?

Answer 19

the amount (in mol0 divided by the volume of the solution

Question 20

What’s a standard solution?

Answer 20

a solution whose concentration is accurately known

Question 21

What are primary standards?

Answer 21

substances used to make a standard solution by weighing

Question 22

Name the characteristics of primary standards

Answer 22

• solids with high molar masses
• available in a high degree of purity
• chemically stable
• not absorb water from the atmosphere
• soluble in water
• react rapidly and completely with other substances when used in titrations

Question 23

State the apparatus needed to make a standard solution

Answer 23

• safety glasses + lab coat
• accurate balance
• weighing bottle
• spatula
• 250cm3 beaker
• 250cm3 volumetric flask
• wash bottle with deionised water
• small funnel
• a glass stirring rod

Question 24

State the apparatus needed for a titration

Answer 24

• a conical flask
• burette (usually 50cm3)
• pipette
• wash bottle with deionised water
• small funnel
• white tile
• clamp stand

Question 25

Give a method for titration

Answer 25

1. rinse conical flask with deionised water and place on white tile
2. rinse pipette with deionised water and then with sodium hydroxide
3. use pipette to transfer 25cm3 sodium hydroxide solution to conical flask
4. add around 3 drops of methyl orange
5. rinse burette with deionised water and then sulfamic acid
6. fill burette with sulfamic solution and set it up in the stand above the conical flask
7. record initial burette reading
8. add sulfamic solution to the flask until indicator just changes colour, record burette reading
9. empty and rinse conical flask with deionised water and repeat titration until concordant titres are obtained

Question 26

Name techniques used in titrations

Answer 26

• white tile
• fill burette so the space between the tap and tip is full of solution
• set burette so tip is in the flask
• record titre to the nearest half a small division e.g. 0.05cm3
• use a light background to see the bottom of the meniscus

Question 27

What’s the equivalence point?

Answer 27

the point at which there are exactly the right amounts of substances to complete the reaction

Question 28

What’s the end point?

Answer 28

the point at which the indicator just changes colour. it should coincide with the equivalence point

Question 29

Define concordant titres

Answer 29

titres which are within 0.20cm3 of each other

Question 30

Define an error

Answer 30

the difference between an experimental value and the accepted or correct value

Question 31

Define accuracy

Answer 31

a measure of how close values are to the accepted or correct value

Question 32

Define precision

Answer 32

a measure of how close values are to each other

Question 33

What’s a random error?

Answer 33

error caused by unpredictable variations in conditions

Question 34

What are systematic errors?

Answer 34

errors that are constant or predictable, usually because of the apparatus used

Question 35

Define the measurement uncertainty

Answer 35

the potential error involved when using a piece of apparatus to make a measurement

Question 36

How do you determine the percentage uncertainty?

Answer 36

the actual measurement uncertainty divided by the value recorded x 100

Question 37

What’s the theoretical yield?

Answer 37

the maximum possible mass of product, assuming complete reaction and no losses

Question 38

What’s the actual yield?

Answer 38

the actual mass obtained

Question 39

How do you work out the percentage yield?

Answer 39

actual yield/theoretical yield x 100

Question 40

What’s the atom economy?

Answer 40

the molar mass of desired mass divided by the sum of molar masses of all products x 100

Question 41

What’s a displacement reaction?

Answer 41

a reaction in which one element replaces another element in a compound

Question 42

What’s the general equation for a metal and acid?

Answer 42

metal + acid —> salt + hydrogen

Question 43

What’s the general equation for a metal oxide and acid?

Answer 43

metal oxide + acid —> salt + water

Question 44

What’s the general equation for a metal hydroxide and acid?

Answer 44

metal hydroxide + acid —> salt + water

Question 45

What’s the general equation for an alkali and acid?

Answer 45

acid + alkali —> salt + water

Question 46

What’s the general equation for a metal carbonate and acid?

Answer 46

metal carbonate + acid —> salt + water + carbon dioxide