9th Class Chemistry Chapter 12 Empirical Data Collection and Analysis Long Questions

Question 1

What are our indigenous units of measurement of mass, volume and length?

Accepted Answer

Before the metric system was adopted in Pakistan, the subcontinent used its own units; some are still heard in markets and villages. Mass Ratti, 8 ratti = 1 masha, 0.12 g; Masha, 12 masha = 1 tola, 0.97 g; Tola, still used for gold and silver, 11.66 g; Chhatank, 5 tola, 58 g; Pao, 4 chhatank = ¼ seer, 233 g; Seer, 16 chhatank = 80 tola, 0.933 kg; Maund (mun), 40 seer, 37.3 kg Volume Liquids such as milk and oil were sold by the seer and pao using standard measuring pots, and the imperial gallon (about 4.55 L) was used for petrol and kerosene before litres. Length Inch (anch), 2.54 cm; Foot (fut), 12 inch, 30.5 cm; Hath (cubit, elbow to fingertip), 18 inch, 46 cm; Gaz (yard), 3 feet, 0.914 m; Kos, about 2-3 km, varied by region Area (land) Marla, kanal, acre, murabba are still used for land (20 marla = 1 kanal; 8 kanal = 1 acre). These units varied from region to region, which is exactly the problem SI solves.

Question 2

Elaborate the difference between precision and accuracy.

Accepted Answer

Accuracy Accuracy measures how close results are to the true or known value. Example: a liquid has a volume of 26 cm³; a student measures 27 cm³ three times; the student is not accurate. Precision The closeness of two or more measurements to each other is called precision. If you weigh a substance five times and get 3.2 kg every time, your measurement is precise, but not necessarily accurate. Independent of each other Precision is independent of accuracy. A student may be accurate but not precise and vice versa. The exact mass of an object is 20 g: Student A (17.3, 17.4, 17.2 g): close together, far from 20 g → precise but not accurate.; Student B (19.8, 20.5, 19.6 g): average ≈ 20.0 g but spread out → more accurate but not precise. Comparison Closeness to the true value., Closeness of readings to each other.; Needs the true value to judge., Judged from repeated readings alone.; Spoiled by systematic error., Spoiled by random error.; Improved by calibration., Improved by careful repeated readings.

Question 3

How can you avoid systematic errors in your measurements?

Accepted Answer

What a systematic error is Errors that naturally occur in measuring tools, affecting the accuracy of the measurement; they may be removed by adding or subtracting a constant adjustment to each measurement. They shift every reading the same way. Ways to avoid them Calibrate instruments against a known standard (check a balance with a standard 100 g mass; a thermometer in melting ice should read 0 °C).; Check and correct the zero error: set the balance to zero (tare) before weighing; subtract any constant offset from every reading.; Use good-quality, graduated glassware (grade-A pipettes and burettes) and clean it, since drops on the walls change the delivered volume.; Use the correct method: read at the bottom of the meniscus at eye level; let hot objects cool before weighing; rinse a burette with the solution it will hold.; Cross-check with a different instrument or method: if two methods agree, a systematic error is unlikely.; Control conditions such as temperature, which changes the volume of glassware and liquids. Repeating readings does NOT remove a systematic error; only calibration or correction does.

Question 4

How does taking measurements in SI units ensure safety and reliability?

Accepted Answer

Safety Medicine doses in mg and mL are understood the same everywhere; mixing units (grains, teaspoons) has caused overdoses.; Engineering and aviation: loads, pressures and fuel in one system prevent disasters. In 1983 an Air Canada jet ran out of fuel because its fuel was worked out in pounds instead of kilograms.; Laboratory safety: concentrations, temperatures and pressures on labels and in instructions are read correctly by everyone. Reliability SI units are based on definite and precise standards, so a measurement means the same in every lab.; A measurement taken in one part of the world can be easily understood and verified in another without confusion, so results can be checked and repeated (reproducibility).; Fewer conversions mean fewer arithmetic mistakes.; Data from many labs can be combined and compared, so conclusions rest on more evidence.

Question 5

Can a student be both inaccurate and imprecise in his measurements?

Accepted Answer

Yes. Precision is independent of accuracy, so all four combinations are possible, including neither accurate nor precise. Example The true mass of an object is 20 g. A student records 17.1 g, 18.9 g and 16.2 g. The readings are far from each other (spread 2.7 g, imprecise) and their average (17.4 g) is far from 20 g (inaccurate). Causes Both a systematic error (e.g. a balance not zeroed, shifting all readings) and random errors (careless reading, air draughts) are present at the same time. Remedy Calibrate and zero the instrument (removes the systematic error), then take several careful readings and average them (reduces random error).

Question 6

(Investigative) Elaborate the importance of using SI units in space exploration.

Accepted Answer

Many countries, one mission Space missions are built by teams in many countries (the International Space Station: USA, Russia, Europe, Japan, Canada). Parts, fuel, forces and distances must be in one system, or they will not fit or work together. A real lesson: Mars Climate Orbiter (1999) NASA lost the Mars Climate Orbiter because one team's software gave thruster impulse in pound-force seconds while the navigation team expected newton seconds (SI). The 4.45-fold error put the spacecraft too close to Mars, and it was destroyed. A mission costing over 300 million dollars was lost to a units mix-up. Why SI matters there Precision: trajectories need exact values of speed (m s⁻¹), mass (kg), thrust (N) and time (s); base-10 SI avoids conversion errors.; Safety of astronauts: oxygen supply, cabin pressure (kPa) and temperature (K) must be read correctly by every crew and ground team.; Data sharing: telescope and probe data in SI can be used directly by scientists everywhere.; Consistency: SI units are interrelated (1 N = 1 kg m s⁻²), so calculations need no hidden conversion factors. Pakistan's space agency SUPARCO, working with China on satellites such as PRSS-1 and the lunar mission iCube-Qamar (2024), also relies on SI units.

Question 7

What are SI units? Why have scientists adopted them? Describe their advantages.

Accepted Answer

Definition Standard and user-friendly units, called SI (Système International) units, that scientists all over the world have agreed to adopt. The problem they solve If scientists in one country measure lengths in metres and in another in feet, then they have to convert. Comparing quantities in different units is not only confusing but a wastage of time as well. Advantages Easy communication worldwide: scientists share data easily.; Fewer conversions are needed to coordinate information.; Base 10, like our number system: easy to learn, remember and convert.; Definite and precise standards.; Interrelated: one unit is derived from others without conversion factors.; Accuracy, consistency and universal understanding: a measurement taken in one part of the world can be understood and verified in another.; Collaboration: scientists compare results, replicate experiments and benefit from each other's work. All of this ensures safety, reliability, reproducibility and progress.

Question 8

Describe the five SI base units used in chemistry with their definitions.

Accepted Answer

There are seven base units in the SI system, out of which chemistry uses five: length, time, amount of substance, mass and temperature. 1. Metre (m), length The distance travelled by light in vacuum in about one 300-millionth of a second (exactly 1/299 792 458 s). 2. Kilogram (kg), mass A block kept in France was taken as the standard unit of mass; it is also described as the mass of 1000 cm³ of water. 3. Second (s), time 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. 4. Kelvin (K), temperature 1/273.16 of the thermodynamic temperature of the triple point of water, where all three states of water exist together. 5. Mole (mol), amount of substance Exactly 6.022 × 10²³ particles of a substance.

Question 9

What are SI prefixes? How are they used? Give examples of conversions.

Accepted Answer

Definition and use Since SI is a metric system based on 10, a set of prefixes indicates whether the unit is a multiple or a fraction of the base ten. It allows the reduction of zeros of a very small or very large number. The prefix symbol comes before the unit symbol. mega, M, 10⁶; kilo, k, 10³; hecto, h, 10²; deca, da, 10¹; deci, d, 10⁻¹; centi, c, 10⁻²; milli, m, 10⁻³; micro, μ, 10⁻⁶; nano, n, 10⁻⁹; pico, p, 10⁻¹² Conversions 2.5 kg → g, 2.5 × 10³, 2500 g; 750 mg → g, 750 × 10⁻³, 0.750 g; 0.25 dm³ → cm³, 0.25 × 1000, 250 cm³; 3.4 MJ → kJ, 3.4 × 10⁶ ÷ 10³, 3400 kJ; 500 nm → m, 500 × 10⁻⁹, 5.00 × 10⁻⁷ m Replace the prefix by its power of ten, then simplify. Write a symbol once even in plural: 5 kg, not 5 kgs.

Question 10

Describe the types of errors in measurement. How can they be reduced?

Accepted Answer

Error Every measurement carries a level of uncertainty known as error: the difference between the measured value and the actual value. It depends on the limitation of the instrument and the skill of the person measuring. 1. Systematic error Errors that naturally occur in measuring tools; they may be removed by adding or subtracting a constant adjustment to each measurement. They affect accuracy. All measuring instruments contribute: a pipette, burette or measuring cylinder may deliver a volume slightly different from the one indicated by its graduation. Reduce: calibrate, set the zero, apply a constant correction, use better instruments. 2. Random error Causes one measurement to differ slightly from the next; comes from unpredictable changes during an experiment: limitations of instruments, environmental factors and slight variation in procedure. It often determines precision. E.g. reading a cylinder from a different angle, air moving the balance. Reduce: repeat readings and average, read at eye level, shield the balance, keep conditions steady. Direction, one way, both ways; Affects, accuracy, precision; Fix, calibration / correction, averaging / care

Question 11

Why do chemists use grams, cubic centimetres and degrees Celsius instead of the SI base units kg, m³ and K?

Accepted Answer

Grams The unit of molar mass is grams per mole; given a mass in grams and the molar mass, we find the moles directly. Laboratory quantities are small, so grams are sensible and give manageable numbers, avoiding excessively large or small values. n = m ÷ M:  4.4 g CO₂ ÷ 44 g mol⁻¹ = 0.1 mol Cubic centimetres cm³ is easy to measure and calculate with, and precise; laboratory volumes are small and more manageable in cm³ than in m³. (1 m³ = 10⁶ cm³.) Degrees Celsius Celsius is more convenient; its 100 divisions fit the base-ten SI format, and it converts easily into kelvin: K = °C + 273. −273, 0 (absolute zero); 0, 273; 25, 298; 65, 338; 100, 373 A temperature change is the same number on both scales: a rise of 10 °C is a rise of 10 K.

Unit	Relation	About
Ratti	8 ratti = 1 masha	0.12 g
Masha	12 masha = 1 tola	0.97 g
Tola	still used for gold and silver	11.66 g
Chhatank	5 tola	58 g
Pao	4 chhatank = ¼ seer	233 g
Seer	16 chhatank = 80 tola	0.933 kg
Maund (mun)	40 seer	37.3 kg

Unit	About
Inch (anch)	2.54 cm
Foot (fut)	12 inch, 30.5 cm
Hath (cubit, elbow to fingertip)	18 inch, 46 cm
Gaz (yard)	3 feet, 0.914 m
Kos	about 2-3 km, varied by region

Accuracy	Precision
Closeness to the true value.	Closeness of readings to each other.
Needs the true value to judge.	Judged from repeated readings alone.
Spoiled by systematic error.	Spoiled by random error.
Improved by calibration.	Improved by careful repeated readings.

Prefix	Symbol	Factor
mega	M	10⁶
kilo	k	10³
hecto	h	10²
deca	da	10¹
deci	d	10⁻¹
centi	c	10⁻²
milli	m	10⁻³
micro	μ	10⁻⁶
nano	n	10⁻⁹
pico	p	10⁻¹²

Change	Working	Answer
2.5 kg → g	2.5 × 10³	2500 g
750 mg → g	750 × 10⁻³	0.750 g
0.25 dm³ → cm³	0.25 × 1000	250 cm³
3.4 MJ → kJ	3.4 × 10⁶ ÷ 10³	3400 kJ
500 nm → m	500 × 10⁻⁹	5.00 × 10⁻⁷ m

	Systematic	Random
Direction	one way	both ways
Affects	accuracy	precision
Fix	calibration / correction	averaging / care

°C	K
−273	0 (absolute zero)
0	273
25	298
65	338
100	373

Long Questions with Answers

Set A: Textbook comprehensive questions 6 questions

What are our indigenous units of measurement of mass, volume and length?

Mass

Volume

Length

Area (land)

Elaborate the difference between precision and accuracy.

Accuracy

Precision

Independent of each other

Comparison

How can you avoid systematic errors in your measurements?

What a systematic error is

Ways to avoid them

How does taking measurements in SI units ensure safety and reliability?

Safety

Reliability

Can a student be both inaccurate and imprecise in his measurements?

Example

Causes

Remedy

(Investigative) Elaborate the importance of using SI units in space exploration.

Many countries, one mission

A real lesson: Mars Climate Orbiter (1999)

Why SI matters there

Set B: More important long questions 5 questions

What are SI units? Why have scientists adopted them? Describe their advantages.

Definition

The problem they solve

Advantages

Describe the five SI base units used in chemistry with their definitions.

1. Metre (m), length

2. Kilogram (kg), mass

3. Second (s), time

4. Kelvin (K), temperature

5. Mole (mol), amount of substance

What are SI prefixes? How are they used? Give examples of conversions.

Definition and use

Conversions

Describe the types of errors in measurement. How can they be reduced?

Error

1. Systematic error

2. Random error

Why do chemists use grams, cubic centimetres and degrees Celsius instead of the SI base units kg, m³ and K?

Grams

Cubic centimetres

Degrees Celsius

The rest of Chapter 12

Long Questions from other Chemistry chapters