Prepare solutions by dissolving or dilution

Ontario Grade 11 Chemistry

Solutions and Solubility

SCH3U expectation E2.3

When salt is stirred into water, the visible crystals disappear and the water becomes a clear solution. The salt has not vanished. Its particles have spread through the water. You can prepare a solution by dissolving a measured amount of solute or by diluting a more concentrated solution. In both methods, measure the final solution volume carefully.

What you will learn

Describe what happens to a solute when it dissolves in a solvent.
Calculate the amount of solute needed to prepare a solution of a stated concentration and volume.
Prepare a diluted solution from a more concentrated solution using the dilution relationship.
Use suitable measuring equipment and record units and significant digits clearly.

1. From a visible substance to a solution

A solution is a uniform mixture. The substance being dissolved is the solute. The substance that dissolves it is the solvent. In salt water, salt is the solute and water is the solvent. The dissolved particles spread throughout the solvent, so samples from different parts of a well-mixed solution have the same concentration.

Volume measures how much space a sample occupies. Solution volumes are commonly measured in litres or millilitres. One litre contains one thousand millilitres, so convert units before combining measurements if needed.

At the particle level, dissolving separates solute particles and disperses them among solvent particles. For an ionic solid such as sodium chloride, the solid separates into sodium ions and chloride ions in water. The ions remain present; they have not been destroyed. The amount of solute is conserved as it dissolves.

1\ \mathrm{L}=1000\ \mathrm{mL}

A solution is a uniform mixture of solute and solvent.
Dissolving spreads solute particles through the solvent.
The amount of solute is conserved when it dissolves.

2. Preparing a solution by dissolving a solid

Concentration describes how much solute is present in a given volume. Molar concentration, written as

c

, is the amount of solute in moles divided by the solution volume in litres. A mole is a counting unit for particles. Molar concentration is measured in moles per litre.

To find the mass of solid needed, use the target concentration and final volume to calculate the amount in moles. Then convert moles to mass using the substance’s molar mass. Molar mass is the mass of one mole of a substance, usually measured in grams per mole. Find it from the chemical formula and the course periodic table.

Weigh the calculated mass and transfer it into a suitable volumetric flask. A volumetric flask has a mark for a precise final volume. Add some solvent and swirl until the solid dissolves. Then add more solvent until the bottom of the liquid curve reaches the mark at eye level. Stopper the flask and mix it by turning it over several times.

The mark indicates the final solution volume, not the amount of solvent added at the start. Dissolve the solid first, then bring the solution to the mark.

c=\frac{n}{V}

Use the target concentration and final solution volume to find the required amount of solute.
Convert the amount in moles to mass when preparing from a solid.
Dissolve the solid first, then bring the solution to its final volume.

3. Preparing a solution by dilution

A dilution makes a solution less concentrated by adding solvent. The starting solution is called the stock solution. The diluted solution has a larger volume and a lower concentration. The amount of solute in the portion transferred from the stock remains the same, provided the transfer is complete.

The dilution relationship compares the concentration and volume before and after dilution. Use

c_1

and

V_1

for the stock concentration and the volume taken from it. Use

c_2

and

V_2

for the final concentration and volume. Keep the volume units consistent. If the concentration units match, both volumes can be in millilitres or both can be in litres.

Measure the required stock volume and transfer it into a volumetric flask. Add solvent until the final-volume mark is reached, then mix thoroughly. The final volume includes both the stock portion and the added solvent.

c_1V_1=c_2V_2

Dilution decreases concentration by adding solvent.
The amount of dissolved solute in the transferred portion remains unchanged.
The final volume includes the stock solution and added solvent.

4. Units, precision, and good preparation

Write units beside measurements throughout a calculation. Check that concentration units match and volume units are consistent. You can use millilitres for both volumes in a dilution calculation, but do not combine a volume in millilitres directly with one in litres without converting.

Significant digits show the precision of a measured or calculated value. Report the final answer to a precision supported by the given data and measuring equipment. Do not report extra decimal places just because a calculator displays them.

Good preparation depends on complete transfer and careful measurement. When preparing from a solid, rinse residue into the flask and mix the solution after reaching the mark. For a dilution, transfer the correct stock volume and mix the final solution. Follow the substance label and appropriate safety directions.

Keep units visible and convert when needed.
Measure the final solution volume, not just the solvent volume.
Report a suitable number of significant digits.

Worked example

Diluting a stock solution

A stock solution has a concentration of

2.00\ \mathrm{mol/L}

. What volume of stock is needed to prepare

250.0\ \mathrm{mL}

of a solution with a concentration of

0.400\ \mathrm{mol/L}

?

Identify the known values
The stock values are $c_1=2.00\ \mathrm{mol/L}$ and unknown $V_1$ . The target values are $c_2=0.400\ \mathrm{mol/L}$ and $V_2=250.0\ \mathrm{mL}$ . The concentration units match, so millilitres can be used for both volumes.
Rearrange the dilution relationship
The transferred stock portion contains the same amount of solute as the final diluted solution. Rearrange the relationship to isolate the stock volume.
$V_1=\frac{c_2V_2}{c_1}$
Substitute values with units
Substitute the target concentration, final volume, and stock concentration. The concentration units cancel, leaving a volume in millilitres.
$V_1=\frac{(0.400\ \mathrm{mol/L})(250.0\ \mathrm{mL})}{2.00\ \mathrm{mol/L}}=50.0\ \mathrm{mL}$
Prepare the diluted solution
Measure $50.0\ \mathrm{mL}$ of stock into a volumetric flask. Add solvent until the total solution volume is $250.0\ \mathrm{mL}$ , then mix well. The stock is already part of the final volume.

Answer: Use

50.0\ \mathrm{mL}

of the stock solution and dilute to a final volume of

250.0\ \mathrm{mL}

.

Check: The stock is five times as concentrated as the target, and

250.0\ \mathrm{mL}

is five times

50.0\ \mathrm{mL}

. The result is consistent.

Common mistakes and how to avoid them

Treating the volume of solvent added as the final solution volume.

Correction: Dissolve the solute or add the stock portion first, then add solvent until the solution reaches the required final volume.

Assuming the amount of solute changes during dilution.

Correction: Adding solvent changes the concentration and volume, but the amount of solute in the transferred stock portion stays the same.

Using litres for one volume and millilitres for the other without conversion.

Correction: Use the same volume unit for both volumes in the dilution calculation, or convert one volume first.

Reporting more precision than the measurements support.

Correction: Round the result to a suitable number of significant digits based on the given values and measuring equipment.

Lesson summary

A solution forms when solute particles spread through a solvent.
To prepare a solution from a solid, calculate the required solute amount, dissolve it, and bring the mixture to the final solution volume.
For dilution, the amount of solute in the transferred stock portion stays constant while solvent is added.
Show units, use consistent volume units, and report appropriate significant digits.

Check your understanding

Question 1

A solid is dissolved to prepare a solution with a stated final volume. When should the solution be brought to that volume?

Before adding the solid
After the solid has dissolved
By measuring only the solvent before adding the solid
After transferring the solid, but before it has dissolved

Show answer and explanation

After the solid has dissolved

Dissolve the solid first, then add solvent until the solution reaches the stated final volume.

Question 2

A student transfers

25.0\ \mathrm{mL}

of stock solution and dilutes it to

100.0\ \mathrm{mL}

. Which statement is correct?

The final solution has the same concentration as the stock.
The final solution is less concentrated, and the transferred solute amount is unchanged.
The final solution is more concentrated because its volume is larger.
The transferred solute amount increases as solvent is added.

Show answer and explanation

The final solution is less concentrated, and the transferred solute amount is unchanged.

Adding solvent increases the solution volume and lowers concentration, while the amount of solute in the transferred portion remains the same.

Question 3

Which volume units can be used directly in a dilution calculation when the concentration units on both sides match?

Millilitres for both volumes
Litres for one volume and millilitres for the other without conversion
Grams for both volumes
Millilitres for one volume and grams for the other

Show answer and explanation

Millilitres for both volumes

Both volumes must use consistent units. Millilitres for both or litres for both are acceptable.

Key terms

Solution: A uniform mixture in which one or more solutes are spread through a solvent.
Solute: The substance that is dissolved.
Solvent: The substance that dissolves the solute.
Molar concentration: The amount of solute in moles divided by the solution volume in litres.
Dilution: The process of lowering a solution's concentration by adding solvent.
Stock solution: A solution with a known concentration that can be used to prepare a more dilute solution.
Volumetric flask: A flask with a calibration mark used to contain a precise final volume.

Continue through SCH3U

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation E2.3. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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