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Questions and Answers
What is the international system of units (SI units)?
It has seven base quantities and base units from which all other quantities can be derived.
Match the following base quantities with their SI units:
Length = Meter Mass = Kilogram Time = Second Temperature = Kelvin Amount of substance = Mole Electric current = Ampere Luminous intensity = Candela
What are the derived SI units for area?
m²
What are the derived SI units for volume?
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Which of the following is NOT a SI prefix?
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What is the conversion for liters to cubic meters?
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The pressure unit atmosphere (atm) equals ______ pascal.
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What is the formula to convert temperature from Kelvin to Celsius?
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Calculate the density in g/cm³ if a substance has a mass of 250 g and its volume is 1500 mL.
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Accuracy is determined from multiple measurements.
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Which of the following defines precision?
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Study Notes
International System of Units (SI Units)
- The SI system is based on seven base quantities and units, from which all other quantities can be derived.
- The seven base quantities include:
- Length - Meter (m)
- Mass - Kilogram (kg)
- Time - Second (s)
- Temperature - Kelvin (K)
- Amount of substance - Mole (mol)
- Electric current - Ampere (A)
- Luminous intensity - Candela (cd)
Derived SI Units
- Derived units are derived from the base units and are used to express other physical quantities.
- Some common derived units include:
- Area - Length2 (m2)
- Volume - Length3 (m3)
- Density - Mass / Volume (kg.m-3)
- Velocity - Distance / Time (m.s-1)
- Acceleration - Velocity / Time (m.s-2)
- Force - Mass x Acceleration (kg.m.s-2 (newton N))
- Pressure - Force / Area (kg.m-1.s-2 (pascal Pa))
- Energy - Force x Distance (kg.m2.s-2 (joule J))
SI Prefixes
- SI prefixes are used to scale units to make them easier to use.
- Common SI prefixes include:
- 106 - Mega (M)
- 103 - Kilo (k)
- 102 - Hecto (h)
- 101 - Deka (da)
- 10-1 - Deci (d)
- 10-2 - Centi (c)
- 10-3 - Milli (m)
- 10-6 - Micro (µ)
- 10-9 - Nano (n)
- 10-15 - Femto (f)
Common Non-SI Units
- Some common non-SI units are still widely used:
- Volume in litres (L)
- 1 mL (cm3) = 10-6 m3
- 1 L = 1000 mL = 1000 cm3 = 1000 x (10-2m)3 = 10-3 m3
- Temperature in Celsius (oC)
- ToK = toC + 273
- toC = ToK - 273
- Pressure in atmosphere (atm)
- 1 atm = 101.325 K.pascal
- Energy in calories (cal)
- 1 cal = 4.148 J
- Volume in litres (L)
Example on Units and Measurement
- A substance with mass = 250 g and volume = 1500 mL has a density of 0.166 g/mL or 0.166 g/cm3.
- In SI units:
- Mass: 250 g = 250 x 10-3 = 0.25 kg
- Volume: 1500 cm3 = 1500 x 10-6 = 0.0015 m3
- Density: 0.25 / 0.0015 = 166.66 kg / m3
Uncertainty of Measurements
- No physical quantity can be measured with perfect certainty.
- There are always errors (uncertainty) in any measurement.
- Each measurement should be reported with some certain digits plus one uncertain digit.
Uncertainty
- Uncertainty depends on the accuracy and precision of the measuring device and the person using it.
Accuracy
- Accuracy measures the closeness of an analytical method's measurement to the accepted true value.
- For example, if a 100.00 g standard mass is weighed and the reading is 78.55 g, the measurement is inaccurate.
Precision
- Precision indicates how close together a group of measurements are.
- Precision is referred to as repeatability or reproducibility.
- Precision requires multiple measurements, while accuracy can be determined with one.
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