GCSE · Chemistry · Edexcel · Spec 1CH0
Concentration of solutions in g/dm3
One spoonful of sugar in a glass of water, five spoonfuls in an identical glass. Same water, same glass, very different drinks. That difference is concentration.
Add water. Count the dots.
Lots of solute particles crowded into this volume. Many particles in the volume means a high concentration.
Each dot is a solute particle in a salt solution (the solvent isn't drawn). Slide to add distilled water and watch what happens to the particles.
Chemistry · Volume units
Concentration wants its volume in dm³. Drag along one whole dm³ and watch the same volume written in cm³, in dm³ and in litres.
Concentration triangle
Tap a quantity to cover it and see how to find it.
Tap the quantity you want to find. The triangle shows you the formula.
Cover mass of solute, concentration or volume of solution to reveal its rearranged formula, then plug in numbers to solve.
Worked example: finding a concentration
Problem
15 g of sodium chloride is dissolved in 0.35 dm³ of water. What is the concentration of the solution in g/dm³? Give your answer to two significant figures.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Add water and the same particles spread out. Then we'll put a number on it, in g/dm³.
What you need to know
- The solute is the substance that dissolves; the solvent is what it dissolves into. In an aqueous (aq) solution, that's water.
- Concentration is the amount of solute dissolved in a certain volume of solvent: solute particles per volume.
- In the same volume, few solute particles means a low concentration and many means a high concentration.
- Adding distilled water dilutes a solution: the volume of solvent grows, the same solute particles spread out, and nothing else is added.
Have a goSam adds distilled water to a beaker of salt solution and announces, “Brilliant, I've got rid of some of the salt.” What has actually happened to the salt particles, and to the concentration?
The salt particles are all still there, just spread through a bigger volume, so the concentration falls.
Distilled water only increases the volume of solvent; no solute is removed. Fewer particles in each bit of volume is what lowers the concentration.
- Concentration and strength are different things in chemistry. Concentration is about particles spread through a specific volume of liquid.
- Volume goes in dm³: 1000 cm³ = 1 dm³ = 1 litre. Divide by 1000 for cm³ to dm³, multiply for dm³ to cm³.
- Mass goes in grams, usually measured on a balance. mg to g: divide by 1000. kg to g: multiply by 1000.
Have a goQuick conversions: turn 750 cm³ into dm³, and 40 mg into g.
750 cm³ = 0.750 dm³ and 40 mg = 0.04 g.
Both conversions divide by 1000: cm³ to dm³ and mg to g. Multiplying by 1000 is for the opposite directions, such as dm³ to cm³ and kg to g.
- Concentration = mass of solute ÷ volume, with mass in g and volume in dm³ giving g/dm³ (also written g dm⁻³).
- Rearranged: volume = mass ÷ concentration, and mass of solute = concentration × volume.
Have a goA solution has a concentration of 5 g/dm³. How many grams of solute are in 3 dm³ of it? Decide which way round the equation goes first.
15 g (5 × 3).
We want the mass, so mass of solute = concentration × volume. The units already match (g/dm³ with dm³), and dividing would give the rearrangement for concentration or volume instead.
- Strategy: choose the equation for what you're finding, convert to g and dm³, then substitute and solve.
The big picture
Concentration says how much solute is dissolved in a certain volume of solvent, so it's about how crowded the solute particles are. Adding distilled water spreads the same particles through more volume, so the concentration falls. In calculations, concentration = mass of solute ÷ volume, with the mass in g and the volume in dm³ to give g/dm³.
Key points
Worked example
Problem
What volume is needed to dissolve 1.78 g of potassium sulfate to give a solution with a concentration of 1.10 g/dm³? Give your answer in dm³ to two significant figures.
⚠ Watch out
Putting a volume in cm³ straight into the equation. Concentration is in g/dm³, so the volume must be in dm³ first. Skip the conversion and your answer is out by a factor of 1000.
Memory hook
Grams on top, dm³ underneath: convert first, then divide.
Check yourself
A solution contains 0.60 g of solute in 300 cm³ of water. Convert the volume first, then find the concentration in g/dm³. (Check: 300 cm³ is 0.300 dm³, and 0.60 ÷ 0.300 = 2.0 g/dm³.)
Flashcards
(12)In a solution, what is the solute and what is the solvent?
What does concentration measure?
Same volume of solvent, more solute particles. What happens to the concentration?
Why does adding distilled water make a solution less concentrated?
Why dilute with distilled water rather than any water?
Are concentration and strength the same thing?
1 dm³ equals how many cm³, and what else is 1 dm³ equal to?
How do you convert cm³ to dm³, and dm³ to cm³?
How do you convert mg to g, and kg to g?
What is the standard unit for concentration, and what does it show?
Write the concentration equation with its units, then both rearrangements.
What are the steps for a concentration calculation?
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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