SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Atomic Foundations of Matter

39 questions · 21 still being checked

Revise, Reflect, Refine 9.8–9.9 (part 11 of 22)

  1. Exercise 9.8

    Determine the formula unit mass of the following substances. (i) Ammonium nitrate (NH4\displaystyle NH_{4}NO3\displaystyle NO_{3}), used as a nitrogen fertiliser, which is essential for plant growth. (ii) Phosphoric acid (H3\displaystyle H_{3}PO4\displaystyle PO_{4}), used to make phosphate fertiliser and detergents. (iii) Sodium hydrogencarbonate (NaHCO3\displaystyle NaHCO_{3}), used to relieve acidity and helps in digestion.
    NCERT’s answer
    (i)
    $\displaystyle 80$ u (ii) $\displaystyle 98$ u (iii) $\displaystyle 84$ u
    (i) Ammonium nitrate, \(\displaystyle NH_4NO_3\) = $\displaystyle 80$ u
    $\displaystyle 2$ N, $\displaystyle 4$ H, $\displaystyle 3$ O: \(\displaystyle (14 \times 2) + (1 \times 4) + (16 \times 3) = 28 + 4 + 48 = 80\) u.
    (ii) Phosphoric acid, \(\displaystyle H_3PO_4\) = $\displaystyle 98$ u
    $\displaystyle 3$ H, $\displaystyle 1$ P, $\displaystyle 4$ O: \(\displaystyle (1 \times 3) + (31 \times 1) + (16 \times 4) = 3 + 31 + 64 = 98\) u.
    (iii) Sodium hydrogencarbonate, \(\displaystyle NaHCO_3\) = $\displaystyle 84$ u
    $\displaystyle 1$ Na, $\displaystyle 1$ H, $\displaystyle 1$ C, $\displaystyle 3$ O: \(\displaystyle 23 + 1 + 12 + (16 \times 3) = 23 + 1 + 12 + 48 = 84\) u.
    Atomic masses used: H = $\displaystyle 1$ u, C = $\displaystyle 12$ u, N = $\displaystyle 14$ u, O = $\displaystyle 16$ u, Na = $\displaystyle 23$ u, P = $\displaystyle 31$ u.

  2. Exercise 9.9

    Write the formulae for the compounds formed by the reaction of: (i) Magnesium and nitrogen (ii) Lithium and nitrogen (iii) Sodium and sulfur (iv) Aluminium and oxygen
    NCERT’s answer
    (i)
    \(\displaystyle Mg_{3}\)\(\displaystyle N_{2}\) (ii) \(\displaystyle Li_{3}\)N (iii) \(\displaystyle Na_{2}\)S (iv) \(\displaystyle Al_{2}\)\(\displaystyle O_{3}\)
    (i) Magnesium and nitrogen → \(\displaystyle Mg_3N_2\) — \(\displaystyle Mg^{2+}\) with \(\displaystyle N^{3-}\); criss-cross $\displaystyle 2$ and $\displaystyle 3$, and \(\displaystyle 3 \times (2+) + 2 \times (3-) = 0\).
    (ii) Lithium and nitrogen → \(\displaystyle Li_3N\) — \(\displaystyle Li^+\) with \(\displaystyle N^{3-}\); three lithium ions balance one nitride ion.
    (iii) Sodium and sulfur → \(\displaystyle Na_2S\) — \(\displaystyle Na^+\) with \(\displaystyle S^{2-}\); two sodium ions balance one sulfide ion.
    (iv) Aluminium and oxygen → \(\displaystyle Al_2O_3\) — \(\displaystyle Al^{3+}\) with \(\displaystyle O^{2-}\); criss-cross $\displaystyle 3$ and 2.
    Nitrogen is not in Table $\displaystyle 9.1$: it has $\displaystyle 5$ valence electrons and, having more than $\displaystyle 4$, gains $\displaystyle 3$ to complete its octet, giving the nitride ion \(\displaystyle N^{3-}\).