Quantitative Chem

Question 1

Law of conservation of mass?

Answer 1

Mass of reactants equals mass of products - hence why we balance chemical equations

Question 2

What are the reasons for masses changing in chemical reactions?

Answer 2

• increase: gas from air became part of product in unsealed container (oxygen can’t be measured mass as a gas but when combined with a solid it can be weighed - e.g. sodium)
• decrease: gas escaped from reaction mixture in a non-closed system

Question 3

Whats a measurement error?

Answer 3

Difference between a result and the EXPECTED value

Question 4

Examples of random and systemic errors in chemistry?

Answer 4

• random: trouble reading scale of instrument correctly, making mistakes when noting readings, environmental changes (e.g. lab gets hotter or air moving around)
• systemic: forgetting to reset the scale to zero, not reading measurement AT the correct eye level

Question 5

What is uncertainty?

Answer 5

How confident experimenter is in measurements- number that says how much the results may be off

Question 6

What is the uncertainty usualy for analogue instruments or digital instruments usually?

Answer 6

• analogue: 1/2 of smallest thing you can measure on it
• digital: the smallest number it can display

Question 7

What does the uncertainty tell us ?

Answer 7

Where the true value is likely to lie

Question 8

Avogadro constant?

Answer 8

6.02 x 10(23) particles per mole

Question 9

What can the particles be in a mole?

Answer 9

Electrons, atoms, molecules or ions

Question 10

What is the mass of 1 mole of any substance?

Answer 10

The relative formula mass in grams

Question 11

formula for moles?

Answer 11

Moles= mass / Mr

Question 12

How to calculate mass of unknown substances?

Answer 12

• identify known (mass) and unknown substances
• find moles of the known substance
• look at the stoichiometry and write the ratio of them
• find the moles of unknown, multiply by its Mr to get mass

Question 13

How to work out balanced symbol equations when you know the masses of the reactants and products?

Answer 13

• find unknown masses using conservation of mass
• find Mr for products and reactants
• Mass/ Mr= mole of each reactant and product
• divide each of the moles by the smallest number out of all of them (whole numbers first)
• use them in front of chemical formulas in equation

Question 14

What are excess of reactants?

Answer 14

• having more of a reactant than needed for reaction
• ensures another reactant is completely used up

Question 15

What is the limiting reactant?

Answer 15

• reactant that is completely used up before the excess reactant
• reaction stops when they get used up (excess reactant remains without reacting

Question 16

What does the amount of product that can be formed in a reaction depend on?

Answer 16

The amount of limiting reactant

Question 17

What should we use when performing reacting mass calculations? Why?

Answer 17

• the limiting reactant
• because it indicates the max amount of product that can be formed

Question 18

Steps for deeming limiting reactant?

Answer 18

1) calculate Mr of each reactant
2) convert mass of each reactant into moles
3) use the molar ratios from the balanced equation

Question 19

Whats a solute?

Answer 19

Solid substance that dissolves in a liquid

Question 20

Whats concentration?

Answer 20

Amount of substance thats in a certain volume of solution (more solute - more concentrated)

Question 21

How many cm3 in a dm3?

Answer 21

1000cm3=1dm3

Question 22

Formula for concentration?

Answer 22

Conc (g/dm3) = mass (g) / volume (dm3)
OR
Conc (mol/dm3)= moles (mol)/ volume (dm3)

Question 23

How to calculate mass of solute?

Answer 23

Use conc equation

Question 24

How to calculate concentration of unknown substance in titrations?

Answer 24

1) find moles of known substance (covert volume to dm3)
2) use the stoichiometry of the balanced equation to find the moles of the known and unknown to write ratio
3) find concentration by diving moles by volume

Question 25

How do we convert between g/dm3 and mol/dm3?

Answer 25

g/dm3 —> mol/dm3: divide by molar mass (Mr) Mol/dm3 —> g/dm3: multiply by molar mass

Question 26

What volume does 1 mol of gas always occupy?

Answer 26

24 dm3 at starts room temp and pressure (20C and 1 atmosphere)

Question 27

Volume of gas equation?

Answer 27

Mol of gas = volume/ 24 dm3

Question 28

How to calculate volume of gases in a reaction using known volume?

Answer 28

Known: unknown (molar ratios) E.g. if its 1:2 - volume of known x 2

Question 29

What is yield?

Answer 29

The amount of product a reaction forms

Question 30

Whats a theoretical yield?

Answer 30

Maximum amount of product estimated to be produced (found using mole calculations)

Question 31

Whats actual yield?

Answer 31

Amount of product that is actually produced in a reaction.

Question 32

Formula for percentage yield?

Answer 32

(Actual / theoretical) x 100

Question 33

Why should industrial processes have as high % yield as possible?

Answer 33

- reduce waste - reduce costs

Question 34

Why is % yield not 100?

Answer 34

Not all reactants react to make product: - reversible reactions product ca turn back into reactant - side reactions with gas in air for example (different than we expect) - loss of product (e.g. remove solid particles from a liquid by filtering you always loose some materials)

Question 35

What is atom economy? What does it tell us?

Answer 35

-Measure of the amount of starting materials (reactants) that end up as USEFUL products - how efficient a reaction is

Question 36

Formula for atom economy?

Answer 36

(Mr of desired products/ Mr of all reactants) x 100

Question 37

Why is height atom economy important?

Answer 37

- high atom economy: economically beneficial since there’s less waste, reducing costs - low atom economy: less sustainable and profitable (waste management and raw material consumption can be costly)