Definition
If every country measured in its own units, scientists would have to keep converting each other's results — causing confusion, mistakes and wasted time. A single agreed system, the International System of Units (SI), lets results be shared, compared and reproduced anywhere.
Systems of units in use: SI, CGS (centimetre–gram–second), MKS (metre–kilogram–second) and the Imperial system.
Remember
SI units are user-friendly because, like our number system, they are based on powers of — so moving from one unit to another needs no awkward conversion factors.
Rule
SI has seven base units; chemistry mainly uses five: length — metre (m), mass — kilogram (kg), time — second (s), temperature — kelvin (K), amount of substance — mole (mol).
One mole of a substance contains particles.
Derived units are built from base units: volume (), density (), area (), force — newton (N), pressure — pascal (Pa ), energy — joule (J ).
Prefixes: mega (M) , kilo (k) , deci (d) , centi (c) , milli (m) , micro () , nano (n) , pico (p)
Example 1 — Celsius to kelvin
Convert to kelvin.
- .
Answer: K
Example 2 — scientific notation
Express in scientific notation.
- Move the decimal point places to the right so that one non-zero digit is before it: .
- Moving right gives a negative power of .
Answer:
Remember
Chemists prefer small units: masses in grams (molar mass is in , so moles are easy to calculate), volumes in rather than (laboratory volumes are small), and temperatures on the Celsius scale, which converts easily to kelvin.
Definition
Error: the difference between the measured (observed) value and the actual value. Every measurement has some uncertainty.
Systematic error: caused by the limited ability of the instrument itself (pipette, burette, cylinder), so every reading is off in the same way. It affects accuracy, and is removed by adding or subtracting a correction to each reading.
Random error: caused by unnoticed changes during the experiment — limits of instruments, small differences in method, changes in surroundings (reading a cylinder from a different angle each time, air disturbing a balance). It is reduced by taking several readings and using their average.
Definition
Accuracy: how close a result is to the actual value.
Precision: how close repeated measurements of the same quantity are to one another.
A result can be precise without being accurate, or accurate without being precise — the two words do not mean the same thing.
Example 3 — accurate or precise?
The actual mass of an object is g. A student measures g, g and g. Comment on the results.
- The three readings are very close to one another, so the work is precise.
- They are all far from the actual value of g, so the work is not accurate.
Answer: Precise but not accurate
Quick Recap
- ●SI units give scientists everywhere one common, reproducible system based on powers of
- ●Five base units used in chemistry: m, kg, s, K, mol; derived units: , , N, Pa, J
- ●; nano , pico
- ●Systematic error: from the instrument, affects accuracy. Random error: unnoticed changes, reduced by averaging
- ●Accuracy: closeness to the actual value. Precision: closeness of repeated readings to each other
