Group 2 metal carbonates thermally decompose to produce a metal oxide and a gas - AQA - GCSE Chemistry Combined Science - Question 6 - 2018 - Paper 1

To find the relative atomic mass (Aₐ) of the Group 2 metal, we start with the formula for the relative formula mass (Mₐ) of the metal carbonate:

egin{align*} Mₐ &= Aₐ + 2 imes 12 + 3 imes 16 \ 197 &= Aₐ + 24 + 48 \ 197 &= Aₐ + 72 \ Aₐ &= 197 - 72 \ Aₐ &= 125 \ ext{Thus, the relative atomic mass of the Group 2 metal is } 125. ext{The Group 2 metal is barium (Ba).} \end{align*}

To calculate the gradient of the line in Figure 8, we use the formula:

Gradient = ( \frac{\Delta Y}{\Delta X} )

From the graph:

• Change in Y (volume of gas) = 300 cm³ - 0 cm³ = 300 cm³
• Change in X (mass of Group 2 carbonate) = 2.0 g - 0 g = 2.0 g

Thus, the gradient is:

$\text{Gradient} = \frac{300}{2.0} = 150 \text{ cm}^3/g$

Unit: cm³/g.

From the information in the question, we know: 24 dm³ of gas corresponds to one mole of the Group 2 carbonate. Using information from the gradient:

• From Figure 8, for every 1.5 g of carbonate, 24,000 cm³ of gas is produced.

Using this value:

• 
  Relative formula mass (Mₐ) = ( \frac{\text{Mass of carbonate}}{ ext{Volume of gas produced}} \times 24 \text{ dm}^3 ) for one mole

Thus:

• ( Mₐ = \frac{1.5 \text{ g}}{24000 \text{ cm}^3} \times 240 \text{ dm}^3 )
• This gives a value around 145 for the relative formula mass of carbonate W.