Question

How many molecules of oxygen are produced when a sample of 38.9 g of water is decomposed by electricity?

A) 6.5 x 10^23
B) 1.3 x 10^24
C) 2.6 x 10^24
D) 2.3 x 10^25

Answer 1

A)
`6.5* 10^(23)\ \text{molecules}`

Step-by-step explanation:

m = Mass of water = 38.9

M = Molar mass of water = 18 g/mol

`N_A` = Avogadro's number =
`6.022* 10^(23)\ \text{mol}^(-1)`

The reaction of electrolysis would be

`2H_2O(l)\rightarrow 2H_2(g)+O_2(g)`

Number of moles of
`H_2O`

`n=(m)/(M)\\\Rightarrow n=(38.9)/(18)\ \text{mol}`

From the reaction it can be seen that 2 moles of
`H_2O` gives 1 mole of
`O_2`

So, number of moles of
`O_2` produced is

`(38.9)/(18)* (1)/(2)=1.081\ \text{mol}`

Number of molecules

`1.081N_A=1.081* 6.022* 10^(23)\\ =6.5* 10^(23)`

So,
`6.5* 10^(23)\ \text{molecules}` of oxygen is produced.