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NCERT Solutions · Class 9 Science Atomic Foundations of Matter

39 questions · 21 still being checked

Pause and Ponder 9.21–9.24 (part 3 of 5)

  1. Exercise 9.21

    Find the molecular mass of nitric acid (HNO3\displaystyle HNO_{3}). Atomic mass — H = 1\displaystyle 1 u; N = 14\displaystyle 14 u; O = 16\displaystyle 16 u.
    NCERT’s answer
    $\displaystyle 63$ u
    Molecular mass of \(\displaystyle HNO_3\) = $\displaystyle 63$ u.
    Add the atomic mass of every atom in the formula: $\displaystyle 1$ H, $\displaystyle 1$ N and $\displaystyle 3$ O.
    \(\displaystyle (1 \times 1) + (14 \times 1) + (16 \times 3) = 1 + 14 + 48 = 63\) u.

  2. Exercise 9.22

    Find the molecular mass of methane (CH4\displaystyle CH_{4}). Atomic mass — C = 12\displaystyle 12 u; H = 1\displaystyle 1 u.
    NCERT’s answer
    $\displaystyle 16$ u
    Molecular mass of \(\displaystyle CH_4\) = $\displaystyle 16$ u.
    The formula holds $\displaystyle 1$ carbon atom and $\displaystyle 4$ hydrogen atoms.
    \(\displaystyle (12 \times 1) + (1 \times 4) = 12 + 4 = 16\) u.

  3. Exercise 9.23

    Find the formula unit mass of potassium chloride (KCl). Atomic mass — K = 39\displaystyle 39 u; Cl = 35.5\displaystyle 35.5 u.
    NCERT’s answer
    74.$\displaystyle 5$ u
    Formula unit mass of KCl = $\displaystyle 74.5$ u.
    \(\displaystyle (39 \times 1) + (35.5 \times 1) = 74.5\) u.
    It is called a formula unit mass, not a molecular mass, because KCl is ionic — its ions form a $\displaystyle 3$-D crystal, so it does not exist as molecules.

  4. Exercise 9.24

    Find the formula unit mass of magnesium hydroxide, Mg(OH)2\displaystyle Mg(OH)_{2}. Atomic mass — Mg = 24\displaystyle 24 u; O = 16\displaystyle 16 u; H = 1\displaystyle 1 u.
    NCERT’s answer
    $\displaystyle 58$ u
    Formula unit mass of \(\displaystyle Mg(OH)_2\) = $\displaystyle 58$ u.
    NCERT_Solution_Class9_Science_Ch9_PP_Q9-24
    The subscript $\displaystyle 2$ outside the bracket multiplies both the O and the H inside it, so the unit contains $\displaystyle 1$ Mg, $\displaystyle 2$ O and $\displaystyle 2$ H.
    \(\displaystyle (24 \times 1) + \{(16 \times 1) + (1 \times 1)\} \times 2 = 24 + (17 \times 2) = 24 + 34 = 58\) u.