Question

Calculate the amount of water formed from thermal decomposition of 9.9 g of zinc hydroxide. What
percentage of mass of reactant gets converted to water?
[Atomic weight of Zn is 65, H is 1 and O is 16)​

Answer 1

Answer:

Explanation:Avogadro's Law states that equal volumes of gases under the same conditions of temperature and pressure contain the same number of molecules.

This means equal amounts of moles of gases occupy the same volume under the same conditions of temperature and pressure.

So the volumes have equal moles of separate particles (molecules or individual atoms) in them.

Therefore one mole of any gas (formula mass in g), at the same temperature and pressure occupies the same volume .

This is 24 dm3 (24 litres) or 24000 cm3, at room temperature of 25oC/298K and normal pressure of 101.3 kPa/1 atmosphere (such conditions are often referred to as RTP).

Sometimes 20oC is treated as room temperature, which means an error of 1.7% on all calculations using the molar gas volume of 24.0 dm3.

The molar volume for s.t.p is 22.4 dm3 (22.4 litres) at 0oC and 1 atmosphere pressure.

Historically, s.t.p unfortunately stands for standard temperature and pressure, but these days 25oC/298K is usually considered the standard temperature (RTP, rtp, r.t.p).

Some handy relationships for substance Z below:

moles Z = mass of Z gas (g) / atomic or formula mass of gas Z (g/mol)

mass of Z in g = moles of Z x atomic or formula mass of Z

atomic or formula mass of Z = mass of Z / moles of Z

1 mole = formula mass of Z  in g.

gas volume of Z = moles of Z x volume of 1 mole

rearranging this equation gives ...

moles of Z = gas volume of Z / volume of 1 mole

moles = V(dm3) / 24   (at RTP)

The latter form of the equation can be used to calculate molecular mass from experimental data because

moles = mass / molecular mass = gas volume / volume of 1 mole

mass / molecular mass = gas volume / volume of 1 mole

molecular mass = mass x volume of 1 mole/volume

therefore at RTP: Mr = mass(g) x 24 / V(dm3)

so, if you know the mass of a gas and its volume, you can work out moles of gas and then work out molecular mass.

This has been done experimentally in the past, but these days, molecular mass is readily done very accurately in a mass spectrometer.

Note (i): In the following examples, assume you are dealing with room temp