46 short questions of 9th Class Chemistry Chapter 12, Empirical Data Collection and Analysis, with exam-ready answers in the book's words: the textbook exercise questions plus SLO-based questions asked in board papers. Punjab Board new syllabus.
Exam-ready answers. Learn the meaning, then write them in your own words.
The short questions at the end of the chapter in your book.
Results are consistent when repeated measurements of the same quantity, made in the same way and in the same units, agree with each other. SI units help consistency: a measurement taken in one part of the world can be easily understood and verified in another part without any confusion.
Example: three titrations giving 24.1, 24.2 and 24.1 cm³ are consistent.
Systematic error: errors that naturally occur in measuring tools; they may be removed by adding or subtracting a constant adjustment to each measurement, and they affect accuracy. E.g. a burette delivering slightly more than its graduation shows.
Random error: the error which causes one measurement to differ slightly from the next; it comes from unpredictable changes during an experiment and determines precision. E.g. reading a cylinder from a different angle each time.
Random errors come from unpredictable changes during an experiment. The main reasons are:
Yes. Systematic error affects the accuracy of the measurement. It shifts every reading in the same direction by the same amount, so the results move away from the true value even if they agree with each other. Example: a balance reading 0.2 g too high gives 5.2 g for a 5.0 g sample every time: precise but not accurate.
Local units are also still used in markets: tola, seer and maund for mass, gaz (yard) for length.
The metre is the standard unit of length, symbol m. It is the distance travelled by light in vacuum in about one 300-millionth of a second (exactly 1/299 792 458 s).
It also saves time and avoids conversion mistakes.
The book's in-text exercises first, then short questions on every topic of the chapter (SLO-based), as asked in board papers.
| Reliable | Reproducible |
|---|---|
| The same person, with the same method and equipment, gets the same result on repeating the experiment. | A different person, in another lab or with different equipment, gets the same result. |
| Tests the consistency of one experimenter. | Tests whether the result holds for everyone. |
SI units are used almost everywhere in the world, giving scientists a single standard for exchanging data. This brings accuracy, consistency and universal understanding. Scientists of different countries and cultures share a common language, so they can compare results, replicate experiments and work together, without confusion or conversion.
(iii) Precise but not accurate. The readings agree exactly (precise), but all are 0.2 g above the true value (not accurate). The constant 0.2 g shift points to a systematic error, such as a balance not set to zero.
Systematic: calibrate instruments against a standard, set the balance to zero, check the zero error and apply a constant correction, use good-quality glassware.
Random: take several readings and use the average, keep the eye level with the mark, shield the balance from air currents, keep conditions (temperature) steady, follow the same procedure each time.
Standard and user-friendly units adopted by scientists all over the world, called the Système International (SI) units. They make communication easy and allow data to be shared.
If scientists in one country measure lengths in metres and in another in feet, they have to convert. Comparing quantities in different units is not only confusing but a wastage of time. Standard units remove this.
Everyone uses the same units, so data from any lab can be used directly; and because SI units are base-10 and interrelated, any conversion needed is only a power of ten.
SI (International System), CGS (centimetre-gram-second), MKS (metre-kilogram-second) and the Imperial system.
Doses, concentrations and pressures read the same everywhere (safety); repeated results agree (reliability); other labs can repeat the work (reproducibility); scientists build on each other's results (progress).
| Quantity | Unit (symbol) |
|---|---|
| Length | metre (m) |
| Time | second (s) |
| Amount of substance | mole (mol) |
| Mass | kilogram (kg) |
| Temperature | kelvin (K) |
Seven: metre, kilogram, second, kelvin, mole, ampere and candela. Chemistry uses five of them.
The standard unit of mass, symbol kg. A block kept in France was taken as the standard; it is also described as the mass of 1000 cm³ of water. (Since 2019 it is defined through the Planck constant.)
The standard unit of time, symbol s: the time that elapses during 9,192,631,770 cycles of the radiation produced by the transition between two levels of the caesium-133 atom.
The standard unit of temperature, symbol K: 1/273.16 of the thermodynamic temperature of the triple point of water, the point at which all the states of water exist at the same time.
The base unit of the amount of a pure substance, symbol mol, defined as exactly 6.022 × 10²³ particles of a substance.
The point at which all three states of water (ice, liquid water and water vapour) exist at the same time: 273.16 K (0.01 °C) at a pressure of 611.657 Pa.
Units of quantities mathematically derived from the base units. Volume: m³ (length³); density: kg m⁻³ (mass ÷ volume); area: m².
| Quantity | Unit | Base units |
|---|---|---|
| Force | newton (N) | kg m s⁻² |
| Pressure | pascal (Pa) | N m⁻² = kg m⁻¹ s⁻² |
| Energy | joule (J) | N m = kg m² s⁻² |
Symbols are not changed in plural forms, so that a symbol always means exactly one thing: 100 mm, not 100 mms ("mms" could be misread).
They indicate whether the unit is a multiple or a fraction of the base ten, and reduce the zeros of very small or very large numbers: 0.000 001 m = 1 μm; 5 000 000 J = 5 MJ.
kilo (k) = 10³, deci (d) = 10⁻¹, milli (m) = 10⁻³. E.g. 1 kg = 1000 g; 1 dm = 0.1 m; 1 mL = 10⁻³ L.
3.5 kg = 3.5 × 1000 = 3500 g. 450 mg = 450 × 10⁻³ = 0.450 g.
1 dm = 10 cm → 1 dm³ = 1000 cm³. 1 m = 100 cm → 1 m³ = 100³ = 10⁶ cm³.
Because the unit of molar mass is grams per mole, so a mass in grams gives the moles directly; and laboratory quantities are small, so grams give manageable numbers.
It is more convenient: it has 100 divisions between the freezing and boiling points of water, compatible with base ten, and it is easy to convert into kelvin (K = °C + 273).
Laboratory volumes are small; cm³ is easy to measure and calculate with, and precise. Burettes, pipettes and cylinders are graduated in cm³.
37 + 273 = 310 K; −10 + 273 = 263 K.
6 500 000 = 6.5 × 10⁶; 0.0072 = 7.2 × 10⁻³.
The difference between the measured value and the actual value. Every measurement carries some uncertainty.
The limitation of the measuring instrument and the skill of the student making the measurement.
A pipette, burette or measuring cylinder delivering a volume slightly different from its graduation; a balance not set to zero; a thermometer reading 1 °C high.
Reading the volume from a measuring cylinder from a different angle each time; a balance giving different values as the surrounding air affects it.
Accuracy measures how close results are to the true or known value. E.g. a liquid of 26 cm³ measured as 27 cm³ is not accurate.
The closeness of two or more measurements to each other. Weighing a substance five times and getting 3.2 kg every time is precise, but not necessarily accurate.
No. Precision is independent of accuracy; a student may be accurate but not precise, and vice versa.
Random error determines precision; systematic error affects accuracy.
Random errors are sometimes high and sometimes low, so they partly cancel in the average, which is closer to the true value. Averaging does not remove a systematic error.
Place it on a level surface, bring the eye level with the bottom of the meniscus (top for mercury), and read the mark that touches it. This avoids parallax error.
