كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

Tools and Inquiry — IB Physics skills

T.1.2 Measuring Variables in Physics — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 3 of 22 1 quiz

📚 IB Physics HL Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

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Lesson summary

T.1.2 Measuring Variables in Physics — IB PHYSICS SL notes: Choose and use instruments to measure mass, time, length, volume, temperature, force, current, voltage, angle, sound and light correctly.Choose an instrument whose range and resolution match the measurement — to give val…

A ruler measures a few centimetres to the nearest millimetre
The instrument must not disturb the system
Liquid thermometer
Digital temperature probe
Stir liquids and keep the bulb away from the walls
Potential difference

Part 1 of 7Choosing instruments

Choosing instruments

Choose an instrument whose range and resolution match the measurement

Choose an instrument whose range and resolution match the measurement to give valid, high-quality data.

A ruler measures a few centimetres to the nearest millimetre

A ruler measures a few centimetres to the nearest millimetre a micrometer suits very small sizes and a tape measure larger distances.

Using the largest possible reading reduces percentage uncertainty.

A 0–10 V meter reading 8.0 V ± 0.1 V has 1.3% uncertainty; reading 0.8 V gives 12.5%.

The instrument must not disturb the system

The instrument must not disturb the system and its sensitivity must match the changes of interest.

Part 2 of 7Mass, time and length

Mass

Mass a digital balance to two decimal places; tare it first and record in kilograms.

Time

Time a stopwatch or stop-clock; the unit is the second.
- (1 minute = 60 s)
Human reaction time matters for intervals under a second — use light gates or video instead.

Length

Length standard unit is the metre; a ruler reads to 1 mm, read perpendicular to avoid parallax.
A micrometer reads to 0.01 mm: use the ratchet and subtract any zero error.
Measure a stack of sheets and divide to reduce percentage uncertainty for thin objects.

Volume

Volume a measuring cylinder; 1 mL of water = 1 cm³.
The change in level gives the volume of an irregular solid.

Part 3 of 7Temperature

Temperature

Liquid thermometer

Liquid thermometer uses expansion of alcohol or mercury in a capillary tube.
Reads to the nearest 1 °C or 0.5 °C; simple, robust and cheap.
It equilibrates slowly and is less precise than digital options.

Digital temperature probe

Digital temperature probe uses thermistors or thermocouples.
Reads to ±0.1 °C or better with fast, real-time readings.
Suits data logging and remote or automated experiments, and reduces human error.

Stir liquids and keep the bulb away from the walls

Stir liquids and keep the bulb away from the walls and read at eye level.

Part 4 of 7Force, current and potential difference

Force, current and potential difference

Force

Force measured directly with a force meter (spring balance) in newtons.

Current

Current an ammeter connected in series with the part measured.

Potential difference

Potential difference a voltmeter connected in parallel across the component.

Check meters for zero error before use

Check meters for zero error before use subtract any zero reading from all measurements.

Analogue meters have a needle and scale and suffer parallax error

Analogue meters have a needle and scale and suffer parallax error read perpendicular to the scale.

Digital meters read small values (mA, µA, mV) and are more precise

Digital meters read small values (mA, µA, mV) and are more precise but the display may flicker, so a judgement is needed.

Typical analogue ranges

0.1–1.0 A and 1.0–5.0 A for ammeters; 0.1–1.0 V or 0–5.0 V for voltmeters.

Part 5 of 7Angle, sound and light

Angle

Angle a protractor with precision ±0.5°.
Align the baseline carefully and read at eye level.
Use a consistent reference point, such as measuring from a drawn normal to the centre of the ray.

Sound intensity

Sound intensity a sound level meter with precision about ±0.1 dB.
Hold at a fixed distance, because intensity falls with distance (inverse square law).
Avoid background noise.

Light intensity

Light intensity a light meter, photodiode or lux meter, precision about ±1 W m^-2.
Keep the detector perpendicular to the source and shield from stray light.
Ambient light affects results, so use a dark room where possible.

Part 6 of 7Reducing measurement errors

Parallax

Parallax view the scale at right angles; a mirror behind the scale helps.

Zero error

Zero error check the reading with no input and subtract it.

Calibration

Calibration compare with a known standard, such as ice at 0 °C and boiling water.

Reaction time

Reaction time time longer intervals or use automated timing.

Environment

Environment shield from draughts, vibration and heat sources when precision matters.

Part 7 of 7More from the specification

1Laboratory Safety with Pressure Equipment

Experiments involving gas pressure and closed thermal vessels require pressure relief valves, protective shielding,
and heat-resistant glass to prevent explosive failure.

2Safe Use and Disposal of Batteries

Chemical battery cells must be monitored against accidental short-circuit heating, stored safely,
and disposed of in designated recycling channels to eliminate toxic heavy metal hazards.

3Apparatus Handling and Risk Assessment

Proper laboratory investigations demand hazard evaluation of high voltages, sharp glassware,
and corrosive reagents, accompanied by personal protective apparatus.

4Ethical Issues in Scientific Experimentation

Researchers must adhere to ethical standards by recording raw data honestly, avoiding fabrication,
and ensuring environmental and biological safety.

5Battery

Two or more cells joined together.

6Apparatus

The equipment needed to carry out an experiment.

7Pressure

Force per unit area, P = F/A.
What the symbols mean: p = pressure (Pa = N m⁻²) · F = force acting at right angles to the surface (N) · A = area (m²)

8Electron diffraction

The spreading of an electron beam by atomic gaps in a crystal, giving ring patterns that show wave behaviour.

Key terms

Term	Meaning
Resolution	Smallest change an instrument can detect or display.
Parallax error	Apparent shift in a reading from viewing a scale at an angle.
Zero error	Non-zero reading when the true value is zero.
Calibration	Checking an instrument against a known standard.
Tare	Setting a balance to zero before weighing.
Reaction time	Delay of about 0.2 s in a human starting or stopping a timer.

Avoid these mistakes

Reading a scale from an angle, causing parallax error.

Ignoring zero error in a micrometer, meter or balance rather than correcting it.

Using a stopwatch for a very short event — reaction time dominates.

Connecting a voltmeter in series or an ammeter in parallel.

Using a coarse instrument whose resolution is too large for the quantity.

Last updated: 15 October 2026