Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.3.1

Units of Volume, Pressure and Temperature; STP and SATP.

How much room does one mole of gas take? 22.414 L, or 24.789 L? Both are right; they were simply measured under two different conditions. Before any gas calculation, then, you need the units of three quantities and the two standard conditions at your fingertips.

1. Volume

Inside a container the gas molecules move about in every direction. The volume within which they move is the volume occupied by the gas. It depends on temperature and pressure.

The SI units of volume are m³, dm³ and cm³; the litre (L) and millilitre (mL) are used too. The relations: 1 m³ = 10³ dm³ = 10³ L = 10⁶ cm³, and 1 L = 1000 mL = 1000 cm³ = 1×10⁻³ m³.

For a conversion, remembering 1 L = 10⁻³ m³ is enough. For example, 22.414 L = 22.414×10⁻³ m³, and 0.5 L = 0.5×10⁻³ m³ = 5×10⁻⁴ m³.

2. Pressure

Molecules keep hitting the container wall as they move. The force this puts on a unit area of the wall is the pressure of the gas. The air pressure at sea level at 45° latitude and 0°C that equals the pressure of a 760 mm column of mercury is one atmosphere, 1 atm.

After Torricelli, the torr is used for low pressures. 1 torr = 1 mm(Hg).

The SI unit of pressure is the newton per square metre, the pascal (Pa). One newton of force on each square metre is one pascal: 1 Pa = 1 N m⁻² = 1 kg m⁻¹ s⁻². And one newton (N) is the force that gives a 1 kg body an acceleration of 1 m s⁻²: 1 N = 1 kg m s⁻².

How many pascals is 1 atm? Take the weight of a 0.76 m column of mercury, with ρ the density of mercury and g the gravitational acceleration: 1 atm = 0.76 m × (13.5951×10³ kg m⁻³) × 9.80665 m s⁻² = 1.01325×10⁵ N m⁻² = 101.325 kPa.

১ atm = ৭৬.০ cm(Hg) = ৭৬০ mm(Hg) = ৭৬০ torr = ১০১.৩২৫ kPa = ১.০১৩২৫×১০⁵ Pa

One needle, four units. Turn the needle, then press STP and SATP.

Every conversion starts from that row. Since 1 atm = 101.325 kPa, 2 atm = 2 × 101.325 = 202.65 kPa, and 760 torr = 101.325 kPa. Turn the cards to test yourself.

3. Temperature

Temperature is measured on three scales: Celsius, Fahrenheit, and the absolute or Kelvin scale. Gas calculations use Celsius and Kelvin.

To go from Celsius to Kelvin, add 273: T = 273 + t. 0°C = 273.15 K, or 273 K for short. 25°C = 298.15 K, or 298 K for short.

Going back, subtract: t = T − 273. So 5 K = 5 − 273 = −268°C, and 298 K = 25°C.

The book takes standard temperature as 0°C or 273 K and room temperature as 25°C or 298 K. Slide the handle to find these points: −273, −268, 0, 25 and 100.

4. STP and SATP

STP (Standard Temperature and Pressure) means a standard temperature of 0°C or 273 K and a standard pressure of 1 atm or 101.325 kPa. Under these conditions the molar volume of a gas is Vm = 22.414 L mol⁻¹.

SATP (Standard Ambient Temperature and Pressure) takes the world-wide room temperature as 25°C or 298 K and the atmospheric pressure as 100 kPa; ambient means surrounding or room. Here the molar volume is Vm = 24.789 L mol⁻¹, so 2 mol of O₂ takes 2 × 24.789 = 49.578 L. Sort each value into the condition it belongs to.

5. Check yourself

Three questions, starting with the first.

In the second, match the unit with the value.

In the last, think of the condition before you answer.

The whole note on eight cards.

Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.3.1.