Module 2.1.3 - Amount of a substance

Question 1

What is the relative molecular mass?

Answer 1

The weighted mean mass of a simple covalent molecule compared with 1/12 of the mass of an atom of carbon-12.

Question 2

How do you calculate the relative molecular mass?

Answer 2

Add up individual relative atomic masses of elements combined within molecule

Question 3

What is the equivalent of relative molecular mass for giant structures (lattices)?

Answer 3

relative formula mass

Question 4

What is amount of a substance used for?

Answer 4

Counting atoms and molecules

Question 5

How is amount of a substance signified and measured?

Answer 5

Signified using symbol, n, and measured using unit of mole

Question 6

What is a mole?

Answer 6

The amount of a substance containing as many particles as there are carbon atoms in exactly 12g of Carbon-12.

Question 7

What is molar mass?

Answer 7

Can be applied to any chemical substance (element,molecule,ion)
Mass of one mole in grams, units are g/mol
Calculated same way as relative molecular/formula masses

Question 8

How is n, mass and relative molecular mass (Mr) related?

Answer 8

amount, n = mass (in grams) / Mr

Question 9

What are the conversions to grams?

Answer 9

1 tonne = 10^6 g
1kg = 1000g
1 microgram = 10^-6 g

Question 10

What is Avogadro’s constant?

Answer 10

The number of particles in one mole of that particle. Given symbol Na.
Value of 6.02 * 10^23 mol. In one mole of any substance there are 6.02 * 10^23 particles

Question 11

How do you work out number of particles?

Answer 11

Na * moles

Question 12

What is stoichiometry?

Answer 12

The ratio between chemical substances in a reaction

Question 13

What is need to calculate reacting masses of solids? (3)

Answer 13

• Fully balanced chemical equations for the reaction
• The mass of one of the reactants/products
• Amount of substance (moles), n = mass/Mr

Question 14

What are the steps in calculating reacting masses? (4)

Answer 14

1. Under balanced equation, fill out the known and unknown values
2. Work out the number of moles in the known substance (using moles equation, exclude balancing number)
3. Work out the moles of the unknown (using molar ratio, aka balancing number)
4. Convert moles to answer the question (using moles equation)

Question 15

What is percentage (%) yield?

Answer 15

A measure of the efficiency of converting reactants into products
Yield - mass of a product obtained from a reaction

Question 16

How do you work out % yield?

Answer 16

Actual mass of product obtained / Theoretical mass of product * 100
OR
Actual mol of product / Theoretical mol of product * 100

Question 17

Why are 100% yields in practice rarely obtained? (3)

Answer 17

• Reaction may be incomplete, as a reversible reaction occurs, and reaction reaches an equilibrium
• Reactants may be impure
• Some reactants/products may be lost when preparing/purifying a compound

Question 18

What are the 4 steps to calculating yield?

Answer 18

1. Calculate the known amount of moles (moles equation) of reactant/product used
2. Use reaction equation to calculate amount in mols of expected amount (theoretical)
3. Convert theoretical moles into theoretical mass
4. Compare actual mass to theoretical mass to calculate the % yield.

Question 19

What is atom economy?

Answer 19

How much of total mass of reactants is converted into desired product.
High atom economy = less waste product

Question 20

How do you calculate atom economy?

Answer 20

INCLUDE BALANCING NUMBERS
molecular mass of the desired product / molecular mass of all products * 100

Question 21

How do you work out the moles in a solution?

Answer 21

Amount, n = concentration (mol/dm^3) * volume (dm^3)

volume must be in dm^3 , if volume in cm^3, n = C*V /1000

Question 22

How do you convert m^3 and dm^3 and cm^3?

Answer 22

1 m3 = 1000 dm3
1 dm3 = 1000 cm3
1 m3 = 10^6 cm3

Question 23

How do you convert between mol dm3 and g dm3?

Answer 23

Mol dm3 * relative formula mass = g dm3

Question 24

What is a standard solution?

Answer 24

A solution of known concentration

Question 25

How do you prepare a standard solution? (5)

Answer 25

1. Accurately weigh the solid (mass by difference method)
2. Dissolve the solid in a beaker using a small amount of distilled water
3. Carefully transfer the solution into the volumetric flask (& rinse beaker with distilled water and add to flask)
4. Fill flask until the bottom of meniscus lines up to the graduation mark
5. Invert flask several times (with stopper) to mix the solution

Question 26

What is molar gas volume, Vm?

Answer 26

The volume per mole of gas molecules at a stated pressure and temperature

Question 27

What is the molar gas volume at RTP?

Answer 27

24.0 dm3/mol OR 24000 cm3/mol (one mole of gas is 24.0 dm3)

Question 28

How do you calculate amount of substance of gases AT RTP?

Answer 28

amount, n = volume (dm3/cm3) / Vm (24.0 dm3/mol or 24000 cm3/mol)

Question 29

What are the two methods of collecting gas?

Answer 29

1. Delivery tube + measuring cylinder in a water tray
2. Collecting a gas with a gas syringe

Question 30

What is the equation to find out volume of gas NOT at RTP?

Answer 30

pV = nRT

p = pressure , Pa
V = volume of gas , m3
n = amount of gas , mol
R = gas constant , 8.314
T = temperature , K

Question 31

What are the conversions needed for ideal gas equation?

Answer 31

1 K Pa = 1000 Pa
1 m3 = 10^-3 dm3 = 10^-6 cm3
Celsius -> kelvin,, add 273

Question 32

What is the empirical formula?

Answer 32

The simplest whole number ratio of atoms of each element present in a compound

Question 33

What is the molecular formula?

Answer 33

The actual number of atoms of each element in a molecule

Question 34

How do you calculate empirical formula?

Answer 34

Find the moles of each element and divide by lowest number of moles. Must be a whole number

(must be +/- 0.1 to be rounded up or down)

Question 35

How do you calculate the molecular formula?

Answer 35

Use the empirical formula and relative molecular mass
Can add all relative atomic masses of all atoms from empirical formula and divide the empirical mass by the molar mass and then multiply the answer

Question 36

What does hydrated mean?

Answer 36

When water of crystallisation is present in a crystal compound

Question 37

What does anhydrous mean?

Answer 37

When all the waters of crystallisation have been removed from a compound

Question 38

What is water of crystallisation?

Answer 38

The water present in a compound giving the compound a crystalline appearance (. x H2O)

Question 39

What is uncertainty?

Answer 39

Each piece of equipment has uncertainty, gives more accuracy. More precise the equipment, lower uncertainty.

Question 40

What assumptions are made about uncertainty on equipment when there is no information present? (2_

Answer 40

• Graduated apparatus: uncertainty assumed to be +/- half of smallest graduation. e.g. graduation 0.1 cm3 -.> uncertainty +/- 0.05 cm3
• Digital apparatus: uncertainty presumed to be resolution of apparatus. e.g 2.d.p balance has uncertainty of +/- 0.01g

Question 41

How do you calculate % error / % uncertainty?

Answer 41

uncertainty / (recorded measurement) * 100

Question 42

When calculating % uncertainty, what do you have to take into account?

Answer 42

How many times the apparatus has been used for measurement.
e.g. temperature (change), titration (burette readings), mass readings -» *2 uncertainty

Question 43

How can % uncertainty be improved (lowered)? (2)

Answer 43

• Increase the mass measurement, percentage uncertainty is lower
• Measure using a more precise equipment (more d.p., +/- 0.001)