SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Atomic Foundations of Matter

39 questions · 21 still being checked

Revise, Reflect, Refine 9.3–9.4 (part 5 of 22)

  1. Exercise 9.3

    You want to design a new ionic compound, where the total positive charge is 6\displaystyle 6+ and the total negative charge is 6\displaystyle 6 -. Which of the following combinations gives the correct number of ions? (i) 2\displaystyle 2 Al3+\displaystyle Al^{3+} and 3\displaystyle 3 Cl (ii) 3\displaystyle 3 Mg2+\displaystyle Mg^{2+} and 1\displaystyle 1 PO4\displaystyle PO_{4} (iii) 2\displaystyle 2 Fe3+\displaystyle Fe^{3+} and 3\displaystyle 3 O2−\displaystyle O^{2-} (iv) 3\displaystyle 3 Ca2+\displaystyle Ca^{2+} and 2\displaystyle 2 SO4\displaystyle SO_{4}

    Not cross-checked

    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    NCERT’s answer
    (iii)
    (iii) $\displaystyle 2$ \(\displaystyle Fe^{3+}\) and $\displaystyle 3$ \(\displaystyle O^{2-}\).
    NCERT_Solution_Class9_Science_Ch9_RRR_Q9-3
    Positive charge: \(\displaystyle 2 \times (3+) = 6+\). Negative charge: \(\displaystyle 3 \times (2-) = 6-\). They balance exactly.
    (i) fails: $\displaystyle 2$ \(\displaystyle Al^{3+}\) gives \(\displaystyle 6+\), but $\displaystyle 3$ \(\displaystyle Cl^-\) gives only \(\displaystyle 3-\).
    (ii) fails: $\displaystyle 3$ \(\displaystyle Mg^{2+}\) gives \(\displaystyle 6+\), but $\displaystyle 1$ \(\displaystyle PO_4^{3-}\) gives only \(\displaystyle 3-\).
    (iv) fails: $\displaystyle 3$ \(\displaystyle Ca^{2+}\) gives \(\displaystyle 6+\), but $\displaystyle 2$ \(\displaystyle SO_4^{2-}\) gives only \(\displaystyle 4-\).
    The compound in (iii) is ferric oxide, \(\displaystyle Fe_2O_3\) — the simplest whole-number ratio of the same two ions.

  2. Exercise 9.4

    Choose the correct statement(s) and correct the false statement(s). (i) Elements are made up of molecules and compounds are made up of atoms. (ii) The molecule of a compound is always made up of two or more atoms of the same kind. (iii) One molecule of nitrogen gas contains three nitrogen atoms. (iv) Water is made of two hydrogen atoms, covalently bonded with one oxygen atom.

    Not cross-checked

    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    NCERT’s answer
    (iv)
    Only (iv) is correct; (i), (ii) and (iii) are false.
    (i) False. Corrected: elements are made up of atoms of one kind (which may join together as molecules of that element, such as \(\displaystyle O_2\)), and compounds are made up of atoms of two or more different kinds, combined as molecules or as ions.
    (ii) False. Corrected: the molecule of a compound is made up of two or more atoms of different kinds — it is a molecule of an element that contains atoms of the same kind.
    (iii) False. Corrected: one molecule of nitrogen gas contains two nitrogen atoms, \(\displaystyle N_2\), joined by a triple bond.
    (iv) True. Oxygen needs $\displaystyle 2$ electrons and each hydrogen needs $\displaystyle 1$, so two hydrogen atoms each share one electron with one oxygen atom — two covalent bonds, giving \(\displaystyle H_2O\).