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$\longrightarrow$ example $NaCl,C _2H _5OH$

(B) The actual number of atoms of each element in one molecule of the substance.

$\longrightarrow$ example $H _2O$ [water]

It shows that water contains $2$ atoms of hydrogen and $1$ atom of oxygen.

(C) The number by parts by weight of the substance [molecular weight]

$\longrightarrow$ Molecular weight can be determined by chemical formula as shown below:-

$CO _2$

one atom of carbon $\times$ mass of carbon + $2$ atoms of oxygen $\times$ mass of oxygen

$=1\times 12+2\times 16$

$=12+32$

$=44g/mol$.

(D) The number of parts by weight of each element $\longrightarrow$ example (1) $C _2H _5OH$ [ethanol] Molecular mass= $46$

(i) Each mole of ethanol contains $2$ moles of carbon

$=12\times2$

$=24g$

(ii) $1$ mole of oxygen atom

$=16\times 1$

$=16g$

(iii) $6$ moles of hydrogen atom

$=1\times 6$

$=6g$

These are present in $46g$ of ethanol

$\therefore$ Percentage of carbon=$\cfrac {24}{46}\times 100=52.17$%

Percentage of oxygen=$\cfrac {16}{46}\times 100=34.78$%

Percentage of hydrogen=$\cfrac {6}{46}\times 100=13.05$%