Quantitative chemistry flashcards

Quantitative chemistry is about using numbers to describe chemical reactions. Students must understand relative atomic mass and relative formula mass, and how to calculate them. The mole is the key unit connecting mass to number of particles, using the formula moles = mass divided by relative formula mass. Students must be able to calculate reacting masses using balanced equations, work out percentage yield and atom economy, and understand why these matter in industrial chemistry. Higher tier students also need to use concentration in mol/dm3 and carry out calculations involving volumes of solutions and gases. Throughout, the ability to rearrange equations and apply them to unfamiliar contexts is essential for success.

1. Card 1

   What is the relative atomic mass (Ar) of an element?

   The relative atomic mass is the average mass of an atom of an element compared to one twelfth of the mass of a carbon-12 atom.

2. Card 2

   How do you calculate the relative formula mass (Mr) of a compound?

   Add together the relative atomic masses of all the atoms shown in the formula. For example, Mr of H2O = (2x1) + 16 = 18.

3. Card 3

   What is a mole in chemistry?

   A mole is the unit chemists use for amount of substance. One mole of any substance contains 6.02 x 10^23 particles, known as the Avogadro constant.

4. Card 4

   State the formula linking moles, mass and relative formula mass.

   moles = mass (g) divided by relative formula mass (Mr). Rearranged: mass = moles x Mr, or Mr = mass divided by moles.

5. Card 5

   How do you use a balanced equation to calculate reacting masses?

   Use the molar ratio from the balanced equation. Convert known mass to moles, scale using the ratio, then convert back to mass for the unknown substance.

6. Card 6

   What is percentage yield and how is it calculated?

   Percentage yield = (actual yield divided by theoretical yield) x 100. It shows how much of the expected product was actually obtained in a reaction.

7. Card 7

   Why is the percentage yield of a reaction rarely 100%?

   Because reactions may be reversible, some product may be lost during transfer or purification, or side reactions may use up some of the reactants.

8. Card 8

   What is atom economy and why does it matter?

   Atom economy = (Mr of desired product divided by total Mr of all products) x 100. High atom economy means less waste and more sustainable industrial processes.

9. Card 9

   How is concentration of a solution calculated?

   Concentration (mol/dm3) = moles divided by volume (dm3). A solution with more moles dissolved in the same volume has a higher concentration.

10. Card 10

    What volume does one mole of any gas occupy at room temperature and pressure?

    At room temperature and pressure, one mole of any gas occupies 24 dm3 (24,000 cm3). This is called the molar volume of a gas.

11. Card 11

    How do you convert cm3 to dm3 when doing concentration calculations?

    Divide the volume in cm3 by 1000 to get dm3. For example, 250 cm3 = 0.25 dm3. This step is essential before using moles = concentration x volume.

12. Card 12

    What does a limiting reactant mean in a chemical reaction?

    The limiting reactant is the substance that is completely used up first, stopping the reaction. It determines the maximum amount of product that can form.

• Students often forget to convert cm3 into dm3 before calculating moles from concentration, leading to answers that are 1000 times too large or too small.
• Many students confuse percentage yield with atom economy, not realising yield is about how much product was collected while atom economy is about the efficiency of the reaction equation itself.
• A common error is using the relative atomic mass instead of the relative formula mass when calculating moles of a compound such as NaCl or H2SO4.

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