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The first four elements in group 1 are lithium, sodium, potassium and rubidium - Edexcel - GCSE Chemistry Combined Science - Question 5 - 2020 - Paper 1
Question 5
The first four elements in group 1 are lithium, sodium, potassium and rubidium. (a) Rubidium reacts with water to form rubidium hydroxide and hydrogen. \[ 2Rb(s) +... show full transcript
Worked Solution & Example Answer:The first four elements in group 1 are lithium, sodium, potassium and rubidium - Edexcel - GCSE Chemistry Combined Science - Question 5 - 2020 - Paper 1
Step 1
Predict what you would see when a small piece of rubidium is placed in a large volume of water.
Answer
When a small piece of rubidium is placed in a large volume of water, you would observe vigorous fizzing and bubbling. This is due to the rapid reaction between rubidium and water, producing hydrogen gas and heat. There may also be splashing of water and, in some cases, the rubidium could ignite in the presence of water due to the exothermic nature of the reaction.
Step 2
Step 3
Calculate the concentration of the rubidium hydroxide solution in g dm⁻³.
Answer
To calculate the concentration of the rubidium hydroxide solution, we first need to determine the number of moles of rubidium used. The molar mass of rubidium (Rb) is 85 g/mol, so:
[ \text{Moles of } Rb = \frac{8.5 ext{ g}}{85 ext{ g/mol}} = 0.1 ext{ moles} ]
Each mole of rubidium produces one mole of rubidium hydroxide (RbOH), thus there are 0.1 moles of RbOH. The concentration can be calculated using the formula:
[ \text{Concentration} = \frac{\text{Moles}}{\text{Volume in dm}^3} ]
Given the volume of the solution is 2.5 dm³:
[ \text{Concentration} = \frac{0.1 ext{ moles}}{2.5 ext{ dm}^3} = 0.04 ext{ mol/dm}^3 ]
To find the concentration in g/dm³, we multiply by the molar mass of RbOH:
[ \text{Concentration in g/dm}^3 = 0.04 ext{ mol/dm}^3 \times 102 ext{ g/mol} = 4.08 ext{ g/dm}^3 ]