Answer to Question #108578 in General Chemistry for Joy

Question

When 20gof compound that contains carbon, hydrogen and oxygen only was burnt in air,29.3gof carbon (iv) oxide and 11.7g of water were produced . calculate the empirical formula of the compound

Answer 1

• Firstly, we should write the equation of organic compound's combustion:

C_xH_yO_z + O₂ $\to$ xCO₂ + y/2H₂O

• According to the equation, we can calculate, for the beginning, amounts of substances of C and H:

$\nu$ (CO₂) = $m/M$ = 29.3(g) / 44(g/mol) = 0.67 mol = $\nu$ (C)

$\nu$ (H₂O) = $m/M$ = 11.7(g) / 18(g/mol) = 0.65 mol $\to$ $\nu$ (H) = 2 ⋅ $\nu$(H₂O) = 2 ⋅ 0.65(mol) = 1.3 mol

• Then, we can calculate the masses of these substances:

m(C) = $\nu ⋅ M$ = 0.67(mol) ⋅ 12(g/mol) = 8.04 g

m(H) = $\nu ⋅ M$ = 1.3(mol) ⋅ 1(g/mol) = 1.3 g

summary mass of m(C) and m(H) = 8.04(g) + 1.3(g) = 9.34(g)

• then m(O) will be calculated this way:

m(O) = $m(compound) - m(summary)$ = 20(g) - 9.34(g) = 10.66(g) $\to$ $\nu$(O) = $m/M$ = 10.66(g) / 16(g/mol) = 0.67 mol

• And now we can determine the empirical formula of the compound, according to this ratio:

$x:y:z = \nu(C): \nu(H):\nu(O)$

x : y : z = 0.67(mol) : 1.3(mol) : 0.67(mol) $\mid$ :0.67(mol)

x : y : z = 1 : 2 : 1

• So, the empirical formula of the compound is H₂CO, which corresponds to formaldehyde

Answer: the empirical formula of the compound is H2CO (formaldehyde)