Foundations of Chemistry. Philippa B. Cranwell

Foundations of Chemistry - Philippa B. Cranwell


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mass of a substance is the amount we measure when we weigh the substance. Mass is a measure of the quantity of a substance. In science, we use the term mass as opposed to weight of a substance to describe the amount because the weight of a substance is related to the gravitational force acting on the substance. We often use the words ‘mass’ and ‘weight’ interchangeably, but they are not really the same thing scientifically.

      The SI unit of mass is the kilogram. However, in the laboratory, chemicals and other objects are usually weighed in units of grams as the gram is a far more convenient amount to work with. Even though we use a balance to weigh a substance, we report its mass in grams, not its weight.

      One kilogram is equivalent to one thousand grams, 1 000 g. 1 kg = 1 000 g. Therefore 1 g =

or 0.001 kg. This can also be written as 1 × 10−3 kg.

      Another common measure of mass is the milligram, mg. One milligram is one‐thousandth of a gram:

      You should always choose a balance appropriate to the level of accuracy of the mass required. If you need ‘about two grams’, for example, a two‐figure balance is perfectly appropriate. If you are told to weigh ‘accurately approximately two grams’, this implies you need to use a four‐figure balance to weigh around two grams of substance but record its weight accurately to four decimal places, e.g. 0.0001 g.

       Volume

      The volume of a substance is the space that it occupies. In chemistry, we are generally most concerned with the volumes of liquids and gases.

If a 250 cm3 measuring cylinder was used to measure out 25 cm3 where the uncertainty was ±0.5 cm3, the percentage uncertainty would be
= 2%. It is therefore more accurate to use a smaller measuring cylinder for this volume of liquid.

      Source: Eisco Labs.

      If we need to know the volume of liquid measured more accurately than this, either a pipette or a burette can be used. A pipette (Figure 0.2b) measures a fixed volume of liquid, and pipettes can be obtained in various sizes. A burette (Figure 0.2c) can be used to deliver variable volumes of liquids; although burettes can be obtained in various sizes, the 50 cm3 burette is the size most commonly used. The uncertainty on a volume measurement when using a laboratory pipette is ±0.06 cm3. This means the uncertainty on measuring a volume of 25 cm3 using a pipette is

= 0.2%. For a standard burette, the uncertainty associated with each reading is ±0.05 cm3. If making one reading, then the percentage uncertainty in measuring 25 cm3 is
0.2%. When recording the amount added from a burette, we make two readings, so the total uncertainty associated with the readings is ±0.1 cm3. Thus the percentage uncertainty on the burette reading is
= 0.4%.

Schematic illustrations of (a) Cube of volume 1 m cubed. (b) Flask of volume 1 L.

      Source: Eisco Labs.

      (c) Cube of volume 1 dm3, not to scale.

      In the chemistry laboratory, we often work with volumes a lot smaller than a cubic metre. A volume you will often encounter is the litre (Figure 0.3b). The symbol that represents the litre is L. However, strictly speaking, this is a non‐SI unit, although it is in common use. Chemists usually work in volumes of cm3 or dm3 where:

equation

      1 dm (decimetre) = 0.1 m (1 × 10−1 m) (Figure 0.3c).

      Remember

      1 dm3 = 1 L = 1 x 10−3 m3 1 000 dm3 = 1 000 L = 1 m3.

      1 dm3 (decimetre cubed) = (0.1 m)3 = ( 1 × 10−1 m)3 = 1 × 10−3 m3.

      1 dm3 is therefore one thousandth of a m3. There are therefore 1 000 dm3 or 1 000 L in 1 m3.

      Imagine the size of a 1 litre carton of orange juice. 1 000 cartons of 1 litre orange juice would fit in a cubic metre.

Photo depicts an orange juice packet. The label has a text that reads, grower's harvest.

      Source: Dr Elizabeth Page.

      Worked Example 0.4

      Convert each of the following masses to grams. You may need to refer to Table 0.3 to obtain the numerical values of the prefixes used. Give your answers in scientific notation:

      1 2.32 Mg

      2 1 000 mg

      3 400 μg


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