Calculations in chemistry

Question 1

What is the symbol for relative formula mass

Answer 1

Mr.

Question 2

How do you calculate the MR of a substance

Answer 2

work out how many atoms of each element there are in the chemical formula

add together the Ar values for all the atoms of each element present

Question 3

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Answer 3

For example, the formula for carbon dioxide is CO2. It consists of one carbon atom (Ar = 12) and two oxygen atoms (Ar = 16):

Mr of CO2 = 12 + 16 + 16 = 44

It could also be calculated this way:

Mr of CO2 = (1 × 12) + (2 × 16) = 12 + 32 = 44

Question 4

Question
Calculate the relative formula mass, Mr, of calcium hydroxide, Ca(OH)2.

(Ar of Ca = 40, Ar of O = 16, Ar of H = 1)

Answer 4

Mr = 40 + (2 × 16) + (2 × 1)

= 40 + 32 + 2

= 74

It could also be calculated this way:

Mr = 40 + 2 × (16 + 1)

40 + 34 = 74

Question 5

Calculate the relative formula mass, Mr, of magnesium nitrate, Mg(NO3)2.

(Ar of Mg = 24, Ar of N = 14, Ar of O = 16)

Answer 5

Mr = 24 + (2 × 14) + (2 × 3 × 16)

= 24 + 28 + 96

= 148

It could also be calculated this way:

Mr = 24 + 2 × [14 + (16 × 3)]

= 24 + 124

= 148

Question 6

Law of conservation of mass

Answer 6

no atoms are lost or made in a chemical reaction. Instead, the atoms join together in different ways to form products
.