GCSE · Chemistry · Edexcel · Spec 1CH0

Concentration in mol/dm3 and conversion from g/dm3

Tip a big scoop of salt into a bath and a pinch into a mug. The bath has far more salt, so is it saltier? Not necessarily.

Chemistry · One solution, two units

Copper sulfate, stated two ways

Concentration: 3/10. Moles per dm³ 0.3 mol/dm³. Mass per dm³ 47.85 g/dm³

Moles per dm³0.3 mol/dm³Mass per dm³47.85 g/dm³

This is ONE copper sulfate solution. The two readouts are the same concentration written in different units, so the g/dm³ number is always 159.5 times the mol/dm³ number (159.5 is the relative formula mass of copper sulfate). Going from g/dm³ to mol/dm³? Divide by 159.5. Going back? Multiply.

Exam line: g/dm³ ÷ relative formula mass = mol/dm³. mol/dm³ × relative formula mass = g/dm³.

Watch out: The 159.5 belongs to copper sulfate. A different solute has a different relative formula mass, so a different exchange rate.

Chemistry · Which is more concentrated?

More solute, more concentrated?

Solution A has 20 g of a solute dissolved in 0.5 dm³. Solution B has 30 g of the same solute dissolved in 1.0 dm³.

Which of these is closest to what you think?

B is more concentrated. It has more solute: 30 g against 20 g.They're equally concentrated, because it's the same solute in both.A is more concentrated. Each dm³ of A holds 40 g, but each dm³ of B holds only 30 g.You can't compare them, because the mass and the volume are both different.

How sure are you?

I'm sureI think soI'm guessing

B is more concentrated. It has more solute: 30 g against 20 g.

This is a common misconception

That compares amounts, not concentrations. Concentration is solute per volume. A is 20 ÷ 0.5 = 40 g/dm³ and B is 30 ÷ 1.0 = 30 g/dm³. A has less solute but packs more of it into every dm³.

They're equally concentrated, because it's the same solute in both.

This is a common misconception

Same solute doesn't mean same concentration. What matters is how much solute is in each dm³. Work each one out: A is 40 g/dm³ and B is 30 g/dm³.

A is more concentrated. Each dm³ of A holds 40 g, but each dm³ of B holds only 30 g.

This matches the evidence

Yes. Divide each mass by its own volume in dm³ (20 ÷ 0.5 = 40 and 30 ÷ 1.0 = 30), then compare. That's the whole idea of a ratio of solute to volume.

You can't compare them, because the mass and the volume are both different.

This is a common misconception

You can, and this is exactly what the ratio is for. Divide each mass by its own volume in dm³ and the two numbers are directly comparable: 40 g/dm³ against 30 g/dm³.

Concentration is solute per volume, not solute on its own.

Two worked calculations

Problem

(1) 15 g of sodium chloride is dissolved in 0.35 dm³. Find the concentration in g/dm³. (2) 0.50 g of copper sulfate (relative formula mass 159.5) is dissolved in 0.25 dm³. Find the concentration in mol/dm³.

Chemistry · Check the working

Find the slip

A student finds the concentration in g/dm³ of 9.0 g of solute dissolved in 300 cm³. Here is their working. Which line goes wrong first?

A student's working — which line goes wrong?

Concentration (g/dm³) = mass in g ÷ volume in dm³
Mass = 9.0 g
Volume = 300 dm³
Concentration = 9.0 ÷ 300
Concentration = 0.030 g/dm³

Cover the one you want

The same shape works for grams: swap moles for mass and mol/dm³ for g/dm³. Match the units before you substitute.

Tap the quantity you want to find. The triangle shows you the formula.

Cover amount of solute, concentration or volume to reveal its rearranged formula, then plug in numbers to solve.

Predict, then check

Two beakers. Each holds 100 cm³ of solution at 0.5 mol/dm³, but the solutes are different, with different relative formula masses.

Which beaker holds more dissolved solute particles?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Same solution, two units, and one number that links them.

What you need to know

Concentration is the amount of solute dissolved in a certain volume: a ratio of solute particles to volume.
In the same volume, more solute means a more concentrated solution.
Have a goSam says: "My bath has 10 spoons of salt in it and your mug has 2, so my bath is saltier." Is that enough to decide? What would you ask Sam first?
No. Ask how much volume each lot of salt is dissolved in.
Concentration is solute per volume, so the amount alone can't settle it. Only in the same volume does more solute mean more concentrated.
You can give concentration in g/dm³ (also written g dm⁻³) or in mol/dm³ (mol dm⁻³).
Divide by the volume: moles ÷ volume in dm³ gives mol/dm³; mass in grams ÷ volume in dm³ gives g/dm³.
Volume must be in dm³. One dm³ is 1000 cm³, so divide a volume in cm³ by 1000.
Have a goYou've got 450 cm³ of solution. What number and unit go into a concentration calculation for the volume?
0.45 dm³
The equations need dm³, and 1 dm³ is 1000 cm³, so you divide cm³ by 1000. Multiplying by 1000 takes you the other way, from dm³ to cm³.
One mole of a solute has a mass equal to its relative mass in grams, so mass = relative mass × moles.
Convert g/dm³ to mol/dm³ by dividing by the solute's relative formula mass, and multiply by it to go back.
Have a goA solution is 90 g/dm³ and the solute's relative formula mass is 18. Which way do you convert to get mol/dm³, and what do you get?
Divide: 90 ÷ 18 = 5 mol/dm³.
g/dm³ to mol/dm³ means dividing by the relative formula mass. Multiplying would be the route from mol/dm³ back to g/dm³.
To get mol/dm³ from a mass, change the mass to moles first, then divide the moles by the volume in dm³.
Rearranged: moles = concentration × volume, mass = concentration × volume, and volume = mass ÷ concentration (match the units first).
Equal volumes of solutions with the same concentration in mol/dm³ hold equal numbers of dissolved solute particles.

The big picture

Concentration tells you how much solute is dissolved in a certain volume, so it's a ratio, not just an amount. You can give it in g/dm³ (mass ÷ volume) or in mol/dm³ (moles ÷ volume), with the volume always in dm³. For one solute these are the same concentration in two units, linked by the solute's relative formula mass: divide by it to go from g/dm³ to mol/dm³, and multiply by it to go back.

Key points

1Concentration = solute per volume: moles ÷ volume in dm³ gives mol/dm³, and mass in grams ÷ volume in dm³ gives g/dm³.

2Volume goes in as dm³: divide cm³ by 1000.

3Relative formula mass is the exchange rate: divide g/dm³ by it to get mol/dm³, multiply mol/dm³ by it to get g/dm³.

4To get mol/dm³ from a mass, do mass to moles first, then moles ÷ volume.

5Equal volumes at the same mol/dm³ hold equal numbers of dissolved solute particles.

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Worked example

Problem

A solution contains 8.0 g of a solute dissolved in 200 cm³. The solute's relative formula mass is 40. Find its concentration in g/dm³ and in mol/dm³.

⚠ Watch out

Thinking more solute automatically means a more concentrated solution. Concentration is solute per volume, so compare the amount in each dm³, not the total amounts. A close cousin: using a volume in cm³ without dividing by 1000.

🧠

Memory hook

Per dm³ is the point, and relative formula mass is the exchange rate: divide by it to get moles, multiply by it to get grams.

✓

Check yourself

Cover the page: a solute has relative formula mass 90, and a solution of it is 45 g/dm³. What is its concentration in mol/dm³, and why did you divide rather than multiply?

Flashcards

(11)

Concentration is a ratio of what to what?

The amount of solute (the number of solute particles) to the volume it is dissolved in.

Which two units can concentration be given in?

g/dm³ (also written g dm⁻³) and mol/dm³ (mol dm⁻³).

How do you calculate concentration in mol/dm³ and in g/dm³?

mol/dm³ = moles ÷ volume in dm³. g/dm³ = mass of solute in grams ÷ volume in dm³.

A volume is in cm³. What do you do before using the concentration equations?

Divide by 1000 to get dm³, because 1 dm³ = 1000 cm³. (Multiply by 1000 to go from dm³ to cm³.)

What is the mass of one mole of a solute?

Its relative mass in grams. So mass in grams = relative mass × moles.

Which way do you use the relative formula mass to convert between g/dm³ and mol/dm³?

g/dm³ to mol/dm³: divide by it. mol/dm³ to g/dm³: multiply by it.

You're given a mass of solute and a volume and need mol/dm³. What are the two stages?

First mass to moles (mass ÷ relative mass), then moles ÷ volume in dm³.

Rearrange the concentration equation to find moles.

moles = concentration (mol/dm³) × volume (dm³). Same shape for mass: mass = concentration (g/dm³) × volume (dm³).

How do you find a volume from a mass and a concentration in g/dm³?

volume = mass ÷ concentration.

Two solutions have equal volumes and the same concentration in mol/dm³. What is equal?

The number of dissolved solute particles.

Why isn't "more solute" enough to say one solution is more concentrated?

Concentration is solute per volume, so the volume matters too. Only in the same volume does more solute mean more concentrated.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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More solute, more concentrated?

Find the slip

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