GCSE · Chemistry · AQA · Spec 8462
Concentration in mol/dm3 (chem HT)
A glass of squash poured from the jug holds far less drink, yet tastes exactly the same. Add water and it tastes weaker. That difference is concentration.
Chemistry · Concentration
This bottle holds a solution labelled 2.0 mol/dm³. The bar is the full 1 dm³ (1000 cm³, the same as 1 litre). Drag to take more or less of it and watch what changes — and what doesn't.
Predict, then check
Careful — this is not the same as pouring some out.
You have a beaker of solution. First you (a) dissolve more solute in it without changing the volume. Then, separately, you (b) add water to a fresh beaker of the same solution, making the volume bigger. What happens to the concentration in (a) and in (b)?
One relationship, three jobs
Tap the quantity you want to find to cover it. What's left tells you whether to multiply or divide.
Tap the quantity you want to find. The triangle shows you the formula.
Cover moles of solute, concentration or volume of solution to reveal its rearranged formula, then plug in numbers to solve.
Titration calculation, step by step
Problem
A titration is used to find the concentration of some sulfuric acid. Each time, 25.0 cm³ of 0.100 mol/dm³ sodium hydroxide solution is exactly neutralised by the acid. The volumes of acid needed (the titres) were: rough 21.20 cm³, then 20.15 cm³, 20.05 cm³ and 20.10 cm³. The equation is 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O. Calculate the concentration of the sulfuric acid in mol/dm³.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
How many moles of solute sit in each 1 dm³ of solution — and how to use that to count moles, find masses and crack titration results.
What you need to know
- Concentration in mol/dm³ is the number of moles of solute in each 1 dm³ of solution.
- 1 dm³ = 1000 cm³, so divide a volume in cm³ by 1000 to get dm³.
- moles of solute = concentration × volume (dm³), and concentration = moles ÷ volume (dm³).
- The mass of one mole of a substance is its relative formula mass (Mr) in grams, so mass = moles × Mr.
- Concentration can also be given in g/dm³: the mass of solute in each 1 dm³. Divide by Mr to turn g/dm³ into mol/dm³.
- In a titration, take the mean of the concordant titres, use the balanced equation's mole ratio, then find the unknown concentration.
The big picture
Concentration in mol/dm³ tells you how many moles of solute are dissolved in each 1 dm³ of solution. Because it is a ratio, pouring out a smaller sample gives you fewer moles but the same concentration, while adding solute or adding water changes it. Moles = concentration × volume in dm³ (so cm³ must be divided by 1000 first), the Mr in grams converts moles to mass, and in a titration you use the concordant mean titre and the balanced equation's mole ratio to find an unknown concentration.
Key points
Worked example
Problem
4.0 g of sodium hydroxide (Mr = 40) is dissolved in water and the solution is made up to 250 cm³. Calculate the concentration of the solution in mol/dm³.
⚠ Watch out
Putting a volume in cm³ straight into moles = concentration × volume. The volume must be in dm³ (divide cm³ by 1000), or every answer comes out 1000 times too big or too small.
Memory hook
The label is 'per 1 dm³'. Pour out less and you get fewer moles, but the label stays the same. Only adding solute or adding water rewrites the label.
Check yourself
Try it: 0.15 mol of solute is dissolved to make 600 cm³ of solution. What is the concentration? (0.25 mol/dm³: 600 cm³ = 0.600 dm³, and 0.15 ÷ 0.600 = 0.25.)
Flashcards
(13)What does a concentration of 1.5 mol/dm³ mean?
What does a concentration in g/dm³ measure?
How many cm³ are in 1 dm³?
How do you work out the moles of solute in a solution?
What is the mass of one mole of a substance?
You know the moles of solute. How do you find its mass?
How do you turn a concentration in g/dm³ into mol/dm³?
You pour 10 cm³ out of a bottle of 0.5 mol/dm³ solution. What is the concentration of your 10 cm³?
What happens to the concentration if you add water to a solution? Why?
Two solutions have the same volume and the same concentration in mol/dm³. What else do they have in common?
What are concordant titres?
Where does the mole ratio for a titration calculation come from?
What is the last step in finding an unknown concentration from a titration?
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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