JEE Main 2019 · Chemistry · s-Block, p-Block Elements and Hydrogen · medium

Phosphorous acid, H3PO3, on heating disproportionates to H3PO4 and PH3. Starting with 8.2 g of H3PO3 (molar mass 82) and complete reaction, the mass…

Single correct option

Phosphorous acid,

\mathrm{H_3PO_3}

, on heating disproportionates to

\mathrm{H_3PO_4}

and

\mathrm{PH_3}

. Starting with

8.2\ \mathrm{g}

\mathrm{H_3PO_3}

(molar mass

82

) and complete reaction, the mass of

\mathrm{H_3PO_4}

(molar mass

98

) formed is

Show answer and solution

Answer: (C) $7.35\ \mathrm{g}$

Balanced equation:

4\mathrm{H_3PO_3} \xrightarrow{\Delta} 3\mathrm{H_3PO_4} + \mathrm{PH_3}

(P goes from

+3

+5

in three atoms and to

-3

in one:

3\times2 = 6

electrons lost,

6

gained). Moles of

\mathrm{H_3PO_3} = 8.2/82 = 0.100

. Moles of

\mathrm{H_3PO_4} = \tfrac34\times0.100 = 0.075

, mass

=0.075\times98 = 7.35\ \mathrm{g}

. Taking a 1:1 ratio would give

9.80\ \mathrm{g}

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