The first four elements in group 1 are lithium, sodium, potassium and rubidium - Edexcel - GCSE Chemistry Combined Science - Question 5 - 2020 - Paper 1

D rubidium has a more exothermic reaction with water than potassium does.

To calculate the concentration, we first need to determine the number of moles of rubidium reacted:

$\text{Moles of } Rb = \frac{8.5 g}{85 g/mol} = 0.1 mol$

The balanced equation shows that for every 2 moles of rubidium, 2 moles of rubidium hydroxide are produced, so:

$\text{Moles of } RbOH = 0.1 mol$

Next, using the volume of the solution (2.5 dm$^3$), we can calculate the concentration:

$\text{Concentration} = \frac{0.1 mol}{2.5 dm^3} = 0.04 mol/dm^3$

To convert moles of rubidium hydroxide to grams, use its molar mass:

$\text{Mass of } RbOH = 0.1 mol \times 102 g/mol = 10.2 g$

Finally, the concentration in g/dm$^3$ is:

$\text{Concentration} = \frac{10.2 g}{2.5 dm^3} = 4.08 g/dm^3$