Molar Concentrations
Übersicht
A technician hands you two identical bottles. Both hold a colourless liquid, both are labelled dilute hydrochloric acid, and both look exactly the same through the glass. One of them would sting for a second on your skin. The other would ruin your laboratory coat. Nothing you can see, smell or weigh from the outside separates them, because what differs is not what the liquid is made of but how tightly the acid is packed into the water. That quantity has a name, a unit and an equation, and without it a bottle of solution is chemically anonymous.
This lesson gives you the number and the method for measuring it. You will learn why chemists count solutions in moles rather than grams, why a careful worker deliberately throws the first result of an experiment away, and how a single drop from a burette can settle a question that no amount of looking at the bottle ever could. At the end you will meet a shortcut that lets you count the particles in a gas simply by measuring the space they occupy, no balance required.
Ziele
- The concentration of a solution is related to the mass of the solute (in terms of number of moles) and the volume of the solution. The concentration of a solution is calculated as follows: Concentration (mol/dm3) = number of moles/volume of solution (in dm3).
- The volumes of acid and alkali solutions that react with each other can be measured by titration using a suitable indicator. Students should be able to carry out titrations using strong acids and strong alkalis only (sulfuric, hydrochloric and nitric acids only). Required practical: Establish the concentration of an unknown strong acid through titration with a strong base.
- If the concentration of one of the reactants is known, the results of a titration can be used to find the concentration of the other reactant. Students should know how to carry out a titration and be able to calculate the chemical quantities in titrations involving concentrations in mol/dm3 and in g/dm3.
- The molar gas volume at room temperature and pressure is assumed to be 24 dm3. Students are expected to be able to calculate the number of moles or the volume of gas in a reaction.
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Unterrichtsbewertung
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- What is 500 cm3 expressed in dm3? A. 0.05 dm3 B. 0.5 dm3 C. 5 dm3 D. 5000 dm3 Answer: B
- A solution contains 0.100 mol of solute in 250 cm3 of solution. What is its concentration? A. 0.025 mol/dm3 B. 0.400 mol/dm3 C. 2.50 mol/dm3 D. 25.0 mol/dm3 Answer: B
- A solution of sodium hydroxide has a concentration of 0.500 mol/dm3. The relative formula mass of NaOH is 40. What is the concentration in g/dm3? A. 0.0125 g/dm3 B. 8.00 g/dm3 C. 20.0 g/dm3 D. 80.0 g/dm3 Answer: C
- What volume is occupied by 2.0 mol of oxygen gas at room temperature and pressure? A. 12 dm3 B. 24 dm3 C. 48 dm3 D. 12 000 dm3 Answer: C
- A student records four titres: 25.30 cm3, 24.45 cm3, 24.50 cm3 and 24.40 cm3. What is the correct mean titre? A. 24.40 cm3 B. 24.45 cm3 C. 24.66 cm3 D. 25.30 cm3 Answer: B
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