Solutions

To fertilise her balcony plants, Nora dissolves $30\ \text{g}$ of soluble fertilizer in water to make $2\ \text{L}$ of solution. Mass concentration is calculated by dividing the grams of solute by the litres of solution. What is the concentration of the fertilizer, in $\text{g/L}$?

To gargle, Marcos dissolves $5\ \text{g}$ of salt in $95\ \text{g}$ of water.

1. a) How many grams of solution does he obtain in total?
2. b) Percentage by mass is calculated by dividing the mass of solute by the total mass of solution and multiplying by $100$. What is the percentage by mass of salt?

A homemade hand sanitiser gel is prepared by mixing $30\ \text{mL}$ of alcohol with aloe vera to make up $150\ \text{mL}$ of gel. Percentage by volume is calculated by dividing the volume of solute by the total volume of solution and multiplying by $100$. What is the percentage by volume of alcohol in the gel?

In the laboratory, you are going to prepare $250\ \text{mL}$ of a solution using a volumetric flask. Put these steps in order and explain each one in your own words:

1. a) Top up with water to the mark on the neck of the flask.
2. b) Weigh the solute on the balance.
3. c) Dissolve the solute in a beaker with a little water.
4. d) Transfer the solution to the flask using a funnel and add water almost up to the mark.

What exactly does "topping up to the mark" mean?

You are asked to prepare $500\ \text{mL}$ of a copper sulphate solution with a concentration of $8\ \text{g/L}$.

1. a) How many grams of copper sulphate should you weigh out?
2. b) Describe how you would prepare the solution in the laboratory, naming the apparatus you would use (balance, beaker, funnel, $500\ \text{mL}$ volumetric flask...) and at what point you top up to the mark.

Three classmates each prepare $250\ \text{mL}$ of a $10\ \text{g/L}$ solution by weighing $2,5\ \text{g}$ of salt, but each makes a mistake:

1. a) Ioana does not tare the balance with the watch glass, so she actually weighs out less salt than she thinks.
2. b) Bruno tops up the flask above the mark on the neck.
3. c) Carla transfers the solution to the flask but forgets to rinse the remaining salt stuck to the beaker with a little water.

For each case, explain what the error consists of and reason whether the actual concentration of their solution is above, below or equal to $10\ \text{g/L}$.

Dry air is a solution of gases: $78\ \%$ of nitrogen, $21\ \%$ of oxygen and $1\ \%$ of argon and other gases, by volume. The figure summarizes this composition.

1. a) What is the solvent in air? Give reasons for your answer.
2. b) Approximately how many litres of oxygen are there in $10\ \text{L}$ of air?

A surface disinfectant label states "$70\ \%$ alcohol by volume". How many millilitres of alcohol does a $250\ \text{mL}$ bottle of this disinfectant contain?

A salt solution has a density of $1,05\ \text{g/mL}$ and a concentration of $10\ \%$ by mass.

1. a) What is the mass of $1\ \text{L}$ of this solution?
2. b) How many grams of salt are in that litre?
3. c) Express the concentration in $\text{g/L}$.
4. d) Is a $10\ \%$ solution by mass the same as a $10\ \text{g/L}$ solution? Give reasons for your answer using the previous results.

For mopping, you dilute $50\ \text{mL}$ of bleach at $40\ \text{g/L}$ in water to make $2\ \text{L}$ of solution. Look at the diagram of the dilution.

1. a) How many grams of hypochlorite are in those $50\ \text{mL}$ of bleach?
2. b) What is the concentration of the solution in the bucket, in $\text{g/L}$?
3. c) Explain why the mass of hypochlorite does not change when adding water, even though the concentration does.

You mix $200\ \text{mL}$ of a $30\ \text{g/L}$ salt solution with $300\ \text{mL}$ of another salt solution at $10\ \text{g/L}$ in the same container. Assume the final volume is the sum of the two. The diagram shows the mixture.

1. a) How many grams of salt does each solution provide?
2. b) What is the concentration of the mixture, in $\text{g/L}$?
3. c) Why is the final concentration not $20\ \text{g/L}$, the mean of the two concentrations?

You prepare solution A by dissolving $18\ \text{g}$ of salt in water to make $300\ \text{mL}$. Then you take $100\ \text{mL}$ of A, pour them into a flask and top up with water to $400\ \text{mL}$: this is solution B.

1. a) Calculate the concentration of A, in $\text{g/L}$.
2. b) How many grams of salt are in the $100\ \text{mL}$ taken from A?
3. c) Calculate the concentration of B from those grams.
4. d) Check that you get the same result using the dilution formula, $c_1\cdot V_1=c_2\cdot V_2$, and explain why it works: what is always conserved when diluting?