Measurement 2 and Experimental Techniques

Mass

• SI unit for mass: $\mathrm{kg}$; smaller masses: $\mathrm{g}$; in industry: $\mathrm{t}$ (tonne).
  • $1\ \mathrm{kg}=1000\ \mathrm{g}$
  • $1\ \mathrm{t}=1000\ \mathrm{kg}$
• Apparatus: beam balance or electronic balance.
• For very small masses: electronic balance with accuracy $\pm 0.01\ \mathrm{g}$.

Time

• SI unit for time: $\mathrm{s}$; longer intervals: $\mathrm{min},\ \mathrm{h}$.
  • $1\ \mathrm{h}=60\ \mathrm{min}$; $1\ \mathrm{min}=60\ \mathrm{s}$.
• Timing devices: stopwatch/stopclock.
  • Analogue stopwatch accuracy: $\pm 0.1\ \mathrm{s}$.
  • Digital stopwatch accuracy: $\pm 0.01\ \mathrm{s}$.
• In chemistry, time is usually recorded to the nearest second.

Temperature

• SI unit: $\mathrm{K}$; also used: $\mathrm{^{\circ}C}$.
• Relation: $T<em>{K}=T</em>{C}+273$ (no degree sign before K).
• Thermometers: mercury or alcohol; each division typically $1^{\circ}\mathrm{C}$.
• Accuracy (thermometers): $\pm 0.5^{\circ}\mathrm{C}$.
• Data loggers: temperature sensor + data logger for more accurate, time-series readings; useful outdoors.

Volume

• SI unit: $\mathrm{m^3}$; also used: $\mathrm{cm^3}$ and $\mathrm{dm^3}$.
  • $1\ \mathrm{m^3}=1000\ \mathrm{dm^3}$
  • $1\ \mathrm{dm^3}=1000\ \mathrm{cm^3}$
  • Hence $1\ \mathrm{m^3}=10^6\ \mathrm{cm^3}$.
• Measuring liquids: various apparatus with different accuracies.
  • Measuring cylinder: reads to nearest $0.5\ \mathrm{cm^3}$ (e.g. 41.5 cm³).
  • Burette: reads to nearest $0.05\ \mathrm{cm^3}$; scale divisions $0.1\ \mathrm{cm^3}$.
  • Pipette: delivers fixed volumes; e.g. $20.0\ \mathrm{cm^3}$ or $25.0\ \mathrm{cm^3}$.
• Gas volumes: measured with gas syringe (max $100\ \mathrm{cm^3}$).

Reading volumes (meniscus)

• When reading a liquid, form a meniscus.
• Read at the bottom of the meniscus with eye level aligned to the liquid surface.
• Example reading: $25.60\ \mathrm{cm^3}$.

Measuring gases

• Gas syringe capacity: maximum volume $100\ \mathrm{cm^3}$.
• Gas collection depends on gas properties:
  • Density relative to air.
  • Solubility in water.
• Three main gas collection methods (Fig. references):
  • Displacement of water: for gases insoluble or slightly soluble in water.
  • Downward delivery: for gases soluble in water and denser than air (e.g., chlorine, hydrogen chloride).
  • Upward delivery: for gases soluble in water and less dense than air (e.g., ammonia).
• Gas density guide: a general rule is that a gas is less dense than air if its molecular mass < 30.
• Drying gases: pass gas through a drying agent.
  • Common drying agents: concentrated sulfuric acid (H$2$SO$4$), quicklime (CaO), fused calcium chloride (CaCl$_2$).
  • Note: H$2$SO$4$ cannot dry ammonia (NH$_3$) because it reacts with it.
• Apparatus examples for drying: ensure the gas passes through drying agent while the exit tube is not immersed in the drying agent.
• Quick reference: dry gases using appropriate drying agent to avoid reactions.

Selecting suitable apparatus for gas collection

• General guide: a gas is less dense than air if its molecular mass is less than $30$.
• Choose collection method by density and solubility:
  • Insoluble or slightly soluble in water: displacement of water.
  • Soluble and denser than air: downward delivery.
  • Soluble and less dense than air: upward delivery.
• When measuring gas volume, gas syringe may be used for controlled volumes up to $100\ \mathrm{cm^3}$.

Quick connections and examples

• Key ideas: measurement decisions depend on accuracy needs and properties of the gas or liquid being measured.
• Important relations:
  • $1\ \mathrm{kg}=1000\ \mathrm{g}$;
  • $1\ \mathrm{t}=1000\ \mathrm{kg}$;
  • $1\ \mathrm{h}=60\ \mathrm{min},\ 1\ \mathrm{min}=60\ \mathrm{s}$;
  • $T<em>K=T</em>C+273$;
  • $1\ \mathrm{m^3}=1000\ \mathrm{dm^3}=10^6\ \mathrm{cm^3}$.