1.3 Chemical calculations

Question 1

Define the relative atomic mass of an element

Answer 1

The average mass of one atom relative to one-twelfth of the mass of one atom of the carbon-12 isotope

Question 2

Define the relative isotopic mass of an isotope

Answer 2

The average mass of an isotope relative to one-twelfth of the mass of one atom of carbon-12

Question 3

How do you calculate the relative atomic mass of two isotopes?

Answer 3

sum of(isotopic mass ×relative abundance) /total relative abundance (usually 100)

Question 4

Define the realative formula/molecular mass of a molecule

Answer 4

The average mass of one molecule relative to one-twelfth of the mass of one atom of carbon-12.

Question 5

How do you work out the percentage of an element in a compound?

Answer 5

(molar mass of element/molar mass of compound) x 100

Question 6

What is the first stage of the mass spectrometer and what does it do?

Answer 6

Vaporisation: the sample is heated and turned into a gas before it enters the spectrometer

Question 7

What is the second stage of the mass spectrometer and what does it do?

Answer 7

Ionisation: the gaseous sample is bombarded with high energy electrons from an electron gun. This knocks electrons off the particles to form positive ions.

Question 8

What is the third stage of the mass spectrometer and what does it do?

Answer 8

Acceleration: the positive ions are accelerated using an electric field and a vacuum pump.

Question 9

What is the fourth stage of the mass spectrometer and what does it do?

Answer 9

Deflection: the ions are deflected by a magnetic field. The amount of deflection depends on the mass and charge of the ion. Lighter ions are deflected more than the heavier ones.

Question 10

What is the fifth stage of the mass spectrometer and what does it do?

Answer 10

Detection: the abundance and mass/charge (m/z) ratio of the ions is read, and a mass spectrum graph is produced.

Question 11

Define a mole

Answer 11

the amount of any substance that contains the same number of particles as there are atoms in exactly 12.0 g of the carbon-12 isotope

Question 12

How do you calculate the number of particles in an element.

Answer 12

no. of moles x avagadros constant (6.02x10^23)

Question 13

What is the equation for moles?

Answer 13

Mr = mass/mol OR conc = mol/vol

Question 14

What are the two standard molar volumes of gas?

Answer 14

• 22.4 dm3 at a temperature of 273K (0 degrees celcius) and a pressure of 1atm
• 24,5 dm3 at a temperature of 298K (25 degrees celcius) and a pressure of 1atm

Question 15

What is the ideal gas equation? and what does each value mean?

Answer 15

PV = nRT

P is the pressure (in Pascal, Pa)
V is the volume (in m3)
n is the number of moles
R is the gas constant (this value is on your data sheet)
T is the temperature (in K).

Question 16

What is the equation to find the number of moles of a gas?

Answer 16

n = volume/molar volume

Question 17

How do we complete a reacting masses calculation?

Answer 17

Mr of unknown compound/Mr of known compound x mass of known compound

Question 18

How do we find out atom economy?

Answer 18

total Mr of required product/ total Mr of reactants x 100

Question 19

What is the equation for percentage error?

Answer 19

estimate of error/value measured x 100