EL - Formulae, equations and amount of substance *01 *02 *03

Question 1

atomic number

Answer 1

the proton number

• all the protons in the nucleus, it defines the element
• all elements of the same atom have the same number of protons

Question 2

mass number

Answer 2

the total number of protons and neutrons on the nucleus

Question 3

what are isotopes?

Answer 3

isotopes of an elements are atoms with the same atomic number but different mass numbers

Question 4

what is relative atomic mass (Ar) ?

Answer 4

the average mass of an atom of an element on a scale where an atom of carbon-12 is 12

Question 5

what is relative isotopic mass?

Answer 5

the mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12.

Question 6

what is relative molecular mass (Mr)?

Answer 6

the average mass of a molecule or formula unit on a scale where carbon-12 is 12

Question 7

How can you work out Ar from mass spectrum?

Answer 7

for each peak: %relative atomic abundance x relative isotopic mass

add all these together

divide by 100

if % not used total abundance might not add up to 100 so need to divide by total relative abundance

Question 8

How can you work out % relative isotopic abundance from a mass spectrum?

Answer 8

= (relative abundance / total relative abundance) x 100

Question 9

what is a mole?

Answer 9

a very large number of particles

1 mole is roughly 6 x 10^23 particles (avogadro’s constant)

Question 10

how do you work out number of moles from the number of particles you have?

Answer 10

no. of mole = no. of particles you have / no. of particles in a mole

eg.
1. 50 x 10^24 of carbon atoms, how many moles?
1. 50 x 10^24 / 6.02 x 10^23 = 2.49 moles

Question 11

what is molar mass? what are the units?

Answer 11

the mass of one mole of something in g mol^-1

it is the same as relative molecular mass

Question 12

how do you work out number of moles when have mass of substance?

Answer 12

moles = mass / Mr (molar mass)

Question 13

moles relationship to concentration equation?

Answer 13

moles = concentration x volume

vol. must be dm^3

Question 14

what is empirical formula?

Answer 14

the smallest whole no. ratio of atoms in a compound

Question 15

what is molecular formula?

Answer 15

the actual no. of atoms in a molecule

• made up of a whole number of empirical units

Question 16

calculating molecular formula?

eg. a molecule has an empirical formula of C4H3O2 and a molecular mass of 166.

Answer 16

1. find relative mass of the atoms in the empirical formula
   (12x4) +(1x3)+(16x2) = 48+3+32 = 83

if molecule mass 166 need to find how many empirical units in it so:
166/83 = 2 empirical units

so molecular formula is emprical formula x 2 = C8H6O4

Question 17

calculating empirical formula from experimental results

eg. when a hydrocarbon is burnt in excess oxygen, 4.4g of CO2 and 1.8g of water are made
what is the empirical formula of the hydrocarbon

Answer 17

1. find moles
   moles of CO2 = 44/12+(16x2) = 0.1 moles
   1 mole of CO2 contains 1 mole of carbon atoms, so must have started with 0.1 moles of carbon atoms
   moles of H2O = 1.8/18 = 0.1 moles
   1 mole of H2O contains 2 moles of hydrogen atoms so must have started with 0.2 moles of H atoms

ratio C:H = 0.1 : 0.2
divide by smallest - 0.1

so: 1 : 2
so empirical formula CH2

Question 18

calculating empirical formula from % of different elements

eg. a compound is found to have the % composition 56.5% potassium. 8.70% carbon and 34.8% oxygen by mass
calculate its empirical formula

Answer 18

use moles = mass / mr

in 100g of compound:

moles of K = 56.5/39.1 = 1.45
moles of C = 8.70/12 = 0.725
moles of O = 34.8/16 = 2.18

divide each of these number by the smallest - 0.725

K = 2.00, C = 1.00, O = 3.01

ratio of K : C : O = 2 : 1 : 3
so empirical formula = K3CO3