2.1.3: amount of substance

Question 1

definition of a mole?

Answer 1

the amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12

Question 2

what is avogadro’s constant?

Answer 2

the number of particles in 1 mole. there are 6.02 x 10(23) atoms in 12 grams of carbon-12, therefore 1 mole of any specified entity contains 6.02 x 10(23) of that entity

can be used for atoms, molecules and ions

Question 3

definition of molar mass?

Answer 3

mass in grams of 1 mole of a substance - given in the unit g mol-1

Question 4

how do you calculate the molar mass of a compound?

Answer 4

add up all the mass numbers of each element in the compound

Question 5

what is molar gas volume?

Answer 5

the volume of 1 mole of a gas at a given temperature and pressure (dm3 mol-1)
all gases have this same volume

Question 6

at what temperature/pressure is the molar gas volume 24 dm3 mol-3?

Answer 6

room temperature = 25•C

room pressure = 1 atm

Question 7

how do you work out the amount of moles in solids?

Answer 7

moles = mass / molar mass (Mr)

e.g. what is the amount, in mol, in 35.0g of CuSO4?
35 / (63.5 + 32 + (16x4))
= 0.219 mol

Question 8

how do you work out the amount of moles in gases?

Answer 8

moles = volume / 24

Question 9

what equation do you use to work out gas volumes that aren’t at room temperature and pressure?

Answer 9

the ideal gas equation

PV = nRT

Question 10

what are the correct units we use for the ideal gas equation?

Answer 10

Pressure = Pa
KPa -> Pa = x1000

Volume = m3
cm3 -> m3 = / 1000000

Temperature = K
C• -> K = +273

n = moles

R = 8.314 J mol-1 K-3

Question 11

how do you work out the amount of moles in solutions?

Answer 11

moles = concentration x volume

Question 12

how do you work out the number of particles in a substance?

Answer 12

number of particles = amount of substance (mol) x avogadro’s constant

e.g. how many atoms of tin are there in a 6.00 g sample of tin metal?
moles = 6/118.7 = 0.05055

atoms = 0.05055 x 6.02 x 10(23)
= 3.04 x 10(22)

Question 13

what is empirical formula?

Answer 13

the simplest ratio of atoms of each element in a compound

Question 14

how do you work out the empirical formula of a compound?

Answer 14

1. divide each mass (or % mass) by the atomic mass of the element
2. divide the answers to step 1 by the smallest answer to get ratio

Question 15

what is molecular formula?

Answer 15

the actual number of atoms of each element in a compound

Question 16

how do you work out molecular formula from empirical formula?

Answer 16

work out how many times the mass of the empirical formula fits into the Mr

e.g. work out the molecular formula for the compound with an empirical formula if C3H6O and a Mr of 116

C3H6O = mass of 58
116 / 58 = 2

molecular formula = C6H12O2

Question 17

what is percentage yield?

Answer 17

how much of the starting materials converted into the desired product (measures the practical efficiency of a process)

Question 18

what does having a high percentage yield mean?

Answer 18

a high conversion of reactions to products

Question 19

how do you work out percentage yield?

Answer 19

% yield =

(actual yield /theoretical yield) x 100

Question 20

25.0g of Fe2O3 was reacted and produced 10g of Fe. what is the percentage yield?
Fe2O3 + 3CO -> 2Fe + 3CO2

Answer 20

1. n(Fe2O3) 25/159.6=0.1566
2. Fe2O3:2Fe
   
   0. 1566:0.313
3. mass(Fe)=0.313x55.8=17.48g
4. (10/17.48)x100=57.2%

Question 21

why would % yield be lowered / why’s it not always possible to get the calculated amount of product in a reaction?

Answer 21

incomplete reactions
side reactions
losses during transfers of substances
losses during purification stages

Question 22

what is atom economy?

Answer 22

the amount of atoms wasted in a reaction - a measure of the proportion of reactant atoms that become part of the desired product

Question 23

what does having a high atom economy mean?

Answer 23

less waste products/wasted atoms

Question 24

how do you work out percentage atom economy?

Answer 24

(Mr of useful products/Mr of all products) x 100

Question 25

what is the % atom economy for the following reaction where Fe is the desired product assuming the reaction goes to completion?
Fe2O3+3CO -> 2Fe+3CO2

Answer 25

(2x55.8)/(2x55.8)+(3x16)+3(12+16)
x 100
=45.8%

Question 26

what is the atom economy for the production of 1,2-dibromoethane (CH2BrCH2Br) in the following reaction:
H2C=CH2 + Br2 -> CH2BrCH2Br

Answer 26

100% because there’s no waste products, only one product

Question 27

what are potential errors in using a gas syringe?

Answer 27

• gas may escape before bung is replaced
• syringe may stick
• some gases like CO2 and SO2 are soluble in water so the true amount of gas isn’t measured

Question 28

what’s the water in a hydrated salt called?

Answer 28

water if crystallisation