At O levels, students must be able to perform calculations involving the number of moles, mass and molar mass of a substance.
To find moles, we use the formula, number of moles = mass/ molar mass. Students need to be aware that the mass should be in grams. Sometimes, certain masses are given in tons, or in kilograms. Before calculating number of moles using the above-mentioned formula, you are to make sure the mass unit is in grams. Hence, it is essential to know how to perform unit conversion for mass.
At O levels, students must be able to perform calculations involving the number of moles, volume and molar volume of a gas. Also, they are required to apply the concept of solution concentration to process the results of volumetric experiments and to solve simple problems. In this two syllabus objectives, students will deal with volume, and are required to know how to convert units of volume.
To find moles, we use the formula, number of moles = volume/ molar volume. Molar volume of a gas at room temperature and pressure is 24 dm3 or 24000 cm3. Some students, when asked to determine number of moles of gas, divide the volume in cm3 by 24, forgetting that the molar volume of 24 is in another unit of dm3. Students must be meticulous and be aware that different units can be used for volume. Being careless and dividing the volumes of different units will cause unnecessary loss of marks in examinations.
When we deal with concentration of solutions, the common unit is mol/dm3. To find mole, students are to multiply concentration in mol/dm3 with volume in dm3. it is necessary to know how to convert from other units to dm3. Common volume units students will encounter at O levels are m3, dm3 and cm3.
In this video, we will go through a simple-to-remember method to convert unit of mass and volume.
Topic: Mole Concept and Chemical Calculations, O Level Chemistry, Singapore