Solutions and their properties

In the laboratory, $0,5\ \text{mol}$ of sodium chloride is dissolved in water to make $2\ \text{L}$ of solution.

1. a) Write down the formula for molarity.
2. b) Calculate the molarity of the solution.

A solution of sodium hydroxide, $\text{NaOH}$, is $0,4\ \text{M}$. Data: molar mass of $\text{NaOH}$, $40\ \text{g/mol}$.

1. a) How many moles of $\text{NaOH}$ are in $250\ \text{mL}$ of that solution?
2. b) How many grams of $\text{NaOH}$ is this?

You are asked to prepare $200\ \text{mL}$ of a $1\ \text{M}$ solution of potassium chloride, $\text{KCl}$. Given: molar mass of $\text{KCl}$, $74,5\ \text{g/mol}$.

1. a) How many moles of $\text{KCl}$ do you need?
2. b) What mass should you weigh?
3. c) Describe the complete laboratory procedure, naming the apparatus (balance, beaker, volumetric flask…) and the moment when it is made up to the mark.

The legal limit of nitrates in drinking water is $50\ \text{ppm}$, i.e. $50\ \text{mg}$ of nitrate per kilogram of water.

1. a) How many milligrams of nitrate, at most, can a container with $5\ \text{kg}$ of water contain?
2. b) In the laboratory, $50\ \text{ppm}$ is often taken as $50\ \text{mg/L}$: explain what is being assumed about the density of water for both ways of expressing it to coincide.

Analysis of a spring finds $30\ \text{mg}$ of dissolved fluoride per $2\ \text{kg}$ of water. The recommended limit for drinking water is $1,5\ \text{ppm}$.

1. a) Express the fluoride concentration of the spring in ppm.
2. b) Does it exceed the limit? How many times does it exceed it?

$10\ \text{mL}$ of a hydrochloric acid solution, $\text{HCl}$, $2\ \text{M}$ are taken and water is added to make up $100\ \text{mL}$ of solution. Observe the diagram of the dilution.

1. a) How many moles of $\text{HCl}$ did the initial $10\ \text{mL}$ contain?
2. b) Calculate the molarity of the diluted solution.
3. c) Verify that the relationship $M_1\cdot V_1=M_2\cdot V_2$ is satisfied and explain which quantity is conserved in a dilution.

A laboratory has nitric acid, $\text{HNO}_3$, $4\ \text{M}$ and needs to prepare $500\ \text{mL}$ of $0,8\ \text{M}$ solution.

1. a) Apply the relationship $M_1\cdot V_1=M_2\cdot V_2$ to calculate the volume of concentrated acid that needs to be measured.
2. b) Describe how to safely prepare the dilution: what apparatus is used and why the acid is added to the water and not the other way round.

In an analytical laboratory, $100\ \text{mL}$ of $0,5\ \text{M}$ $\text{NaCl}$ solution are mixed with $300\ \text{mL}$ of $1,5\ \text{M}$ $\text{NaCl}$ solution. Assume that the volumes are additive. The diagram shows the mixture.

1. a) Calculate the moles of salt provided by each solution.
2. b) Calculate the molarity of the resulting mixture.
3. c) Explain why the result is not $1\ \text{M}$, the mean of the two initial concentrations.

The colligative properties of solutions depend on the number of dissolved solute particles and not on their nature.

1. a) Order from lowest to highest freezing point: pure water, $0,5\ \text{M}$ salt solution, and $1\ \text{M}$ salt solution. Explain your answer.
2. b) Order those same three samples from lowest to highest boiling point.
3. c) Does it affect these orderings whether the solute is salt or sugar? Explain what it means for them to be colligative properties.

Two compartments are separated by a semipermeable membrane, which allows water to pass through but not glucose. In one there is a $0,1\ \text{M}$ glucose solution and in the other a $1\ \text{M}$ solution. The diagram shows the initial situation.

1. a) Explain what osmosis is.
2. b) Predict which way the water will pass through the membrane and justify your answer.
3. c) What difference will be observed between the two compartments over time?
4. d) What is osmotic pressure? State what would need to be done to prevent this flow of water.

Red blood cells are surrounded by a semipermeable membrane and contain an aqueous medium with dissolved salts and proteins.

1. a) Explain why saline or intravenous fluid injected into a vein must be isotonic, that is, have a solute concentration similar to that of blood. Which colligative phenomenon is involved?
2. b) Predict what would happen to red blood cells if the IV fluid were much more dilute than blood: which way would water flow and what risk would there be?
3. c) What if it were much more concentrated? Justify your answer in the same way.