2.1.3 Amount of substance

Question 1

What is the formula for calculating the percentage of an element in a compound?

Answer 1

% of an element = (number of atoms X relative atomic mass ÷ Mr) x 100

Question 2

What is the equation for working out the number of atoms/ions/molecules in a substance?

Answer 2

Number of atoms/ions/molecules = Moles x 6.02x10²³

Question 3

What is the method for calculating the empirical formula?

Answer 3

• Find the mass of both substances (usually given in the question)
• Calculate the moles of both substances
• Divide the mole values by the smallest value
• Adjust the ratio. If the compound is out by 0.01, it can be rounded. If it isn’t, the compound must be scaled up.
• Form the formula from the ratio

Question 4

Non-required practical

Describe the method for calcualting the amount of water in a hydrated salt.

Answer 4

• Find the mass of the anhydrous salt by taking away the mass of the crucible
• Find the mass of water by taking away the hydrated crystals from the anhydrous
• These 2 values are the masses which can be subsituted into the emprical formula method.

Question 5

What is the equation for molar gas volume?

Answer 5

Moles (n) = volume (dm³) / 24

24,000 should be used if the volume is given in cm³

Question 6

What is the ideal gas equation?

Answer 6

pV = nRT

• P = Pressure (Pa)
• V = volume (m³)
• n = moles of gas (mol)
• R = ideal gas constant (8.314Jmol⁻¹K⁻¹)
• T = temprature (K)

Question 7

How is Kelvin (K) converted into ℃?

Answer 7

+273

Question 8

How is KPa converted into Pa?

Answer 8

Take the KPa value and add x10³ onto it

Question 9

What is the equation for concentration?

Answer 9

Concentration = (moles X 1000) ÷ volume

Question 10

What is the equation for % yield?

Answer 10

(Actual moles ÷ theoretical moles) x100

Question 11

What is the equation for atom economy?

Answer 11

(Mr of desired products ÷ Mr of all products) x100