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Unit 2 of 4 · B.Sc MLS Sem 1

Unit 2: Water and units of measurement

Basics of Biochemistry notes · PTU syllabus (BMLS103-18)

3 min read4 topics9 exam questions
On this page
  1. Unit summary
  2. Distilled water
  3. SI and CGS units
  4. Molecular weight and equivalent weight
  5. Normality, molarity and molality
  6. Key terms
  7. Quick revision
  8. Important questions

Unit summary

Pure water and correct units are the foundation of every reagent. This unit covers the preparation and storage of distilled water and types of distillation plants, SI and CGS units and their conversion, and expressions of strength — molecular weight, equivalent weight, normality, molarity and molality.

After this unit you can

  • Prepare and store distilled water
  • Use SI and CGS units and convert between them
  • Calculate molecular and equivalent weights
  • Calculate normality, molarity and molality

PTU syllabus topics

  • Distilled water preparation and types of water distillation plants
  • storage of distilled water
  • S.I. and CGS units
  • conversion
  • strength
  • molecular weight
  • equivalent weight
  • normality
  • molarity
  • molality
Key formulasExpressing concentration

  • Molarity (M)

    Moles of solute / litres of solution

  • Normality (N)

    Gram equivalents / litres of solution

  • Molality (m)

    Moles of solute / kg of solvent

  • Equivalent weight

    Molecular weight / n-factor

  • Dilution

    C1 V1 = C2 V2

1

Topic 1

Distilled water

  • Distillation: water is boiled and the steam condensed, leaving dissolved salts, organic matter and most microbes behind.
ComparisonWater purification
Method
Purity

Single distillation

Boil and condense once

Routine reagent water

Double (glass) distillation

Distil again in all-glass apparatus

Higher purity for sensitive tests

Deionisation

Ion-exchange resins remove ions

Low conductivity; organics may remain

Reverse osmosis

Pressure through a semipermeable membrane

Removes most ions, organics, microbes

Ultrapure (type I)

RO + deionisation + UV + filtration

Molecular biology, HPLC

Key termsDistillation plants
Metal (stainless steel or copper) still
Large volumes; tinned copper or steel
All-glass still
Quartz or borosilicate; avoids metal ions
Double distillation unit
Two stills in series
  • Storage: clean, tightly closed polyethylene or borosilicate containers; avoid long storage — water absorbs CO₂ (becoming slightly acidic) and can grow bacteria. Check conductivity regularly.
2

Topic 2

SI and CGS units

ComparisonUnits
SI unit
CGS unit

Length

metre (m)

centimetre (cm)

Mass

kilogram (kg)

gram (g)

Time

second (s)

second (s)

Amount of substance

mole (mol)

—

Volume

cubic metre; litre (L) commonly used

cubic centimetre (cc = mL)

Key termsPrefixes
kilo (k)
10³
deci (d)
10⁻¹
centi (c)
10⁻²
milli (m)
10⁻³
micro (µ)
10⁻⁶
nano (n)
10⁻⁹
Key formulasConversions in clinical chemistry

  • mg/dL to mmol/L

    mmol/L = mg/dL × 10 ÷ molecular weight

  • Glucose

    90 mg/dL × 10 ÷ 180 = 5.0 mmol/L

  • Urea

    Urea (mg/dL) = BUN × 2.14

3

Topic 3

Molecular weight and equivalent weight

  • Molecular weight: sum of the atomic weights of the atoms in a molecule (NaCl = 23 + 35.5 = 58.5).
Key formulasEquivalent weight

  • Acid

    Molecular weight ÷ basicity (replaceable H⁺)

  • Base

    Molecular weight ÷ acidity (replaceable OH⁻)

  • Salt

    Molecular weight ÷ total valency of cation

  • Examples

    HCl 36.5 ÷ 1 = 36.5; H₂SO₄ 98 ÷ 2 = 49; NaOH 40 ÷ 1 = 40; Na₂CO₃ 106 ÷ 2 = 53

4

Topic 4

Normality, molarity and molality

Key formulasConcentration

  • Molarity (M)

    Moles of solute per litre of solution = grams ÷ (molecular weight × litres)

  • Normality (N)

    Gram equivalents per litre of solution = grams ÷ (equivalent weight × litres)

  • Molality (m)

    Moles of solute per kg of solvent

  • Relation

    N = M × n (n = basicity, acidity or valency)

  • Percentage

    w/v = g per 100 mL

Example

0.1 N NaOH: 40 × 0.1 = 4 g in 1 L. 0.1 M H₂SO₄: 98 × 0.1 = 9.8 g per litre (= 0.2 N). 0.2 M Na₂CO₃: 106 × 0.2 = 21.2 g per litre.

  • Molality does not change with temperature (it uses mass), whereas molarity and normality do (volume changes).

Key terms

Distilled water
Water purified by boiling and condensing
Molarity
Moles of solute per litre of solution
Normality
Gram equivalents of solute per litre of solution
Molality
Moles of solute per kilogram of solvent
Equivalent weight
Mass reacting with or replacing one mole of hydrogen

Quick revision

  • Distillation, double distillation, deionisation, RO, ultrapure water; storage.
  • SI and CGS units; prefixes; mg/dL ↔ mmol/L.
  • Molecular weight; equivalent weight of acids, bases, salts.
  • M, N, m; N = M × n; percentage solutions.

Important exam questions

Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).

Short-answer questions

  1. Q1.Why is double-distilled water used?
  2. Q2.Why does stored distilled water become acidic?
  3. Q3.Convert glucose 126 mg/dL to mmol/L.
  4. Q4.Find the equivalent weight of H₂SO₄.
  5. Q5.How much NaOH is needed for 500 mL of 0.1 N solution?
  6. Q6.Distinguish molarity and molality.

Long-answer questions

  1. Q1.Describe the preparation and storage of distilled water and types of distillation plants.
  2. Q2.Explain SI and CGS units with conversions.
  3. Q3.Explain normality, molarity and molality with calculations.

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