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

The correct answer is D: rubidium has a more exothermic reaction with water than potassium does. This is indicative of its greater reactivity, as it indicates that the reaction releases more energy compared to potassium.

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

1. Calculate the number of moles of rubidium:

   $ext{moles} = \frac{\text{mass}}{\text{molar mass}} = \frac{8.5 g}{85 g/mol} = 0.1 mol$

2. Since the reaction produces 2 moles of RbOH for every mole of Rb, the moles of RbOH produced will also be 0.1 mol.

3. Now, calculate the concentration using:

   $\text{concentration (g/dm}^3\text{)} = \frac{\text{mass of solute (g)}}{\text{volume of solution (dm}^3\text{)}}$

4. The mass of RbOH produced is:

   $\text{mass} = \text{moles} \times \text{molar mass} = 0.1 mol \times 102 g/mol = 10.2 g$

5. Therefore, the concentration is:

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