Chemistry

(c) 8

Nitrogen goes from $+5$ in $NO_3^-$ to $-3$ in $NH_3$, a gain of 8 electrons. Balanced: $NO_3^-+6H_2O+8e^-\to NH_3+9OH^-$.

(d) 3

Mn: $[Ar]3d^54s^2$. $Mn^{4+}$ loses two 4s and two 3d electrons giving $3d^3$, which has 3 unpaired electrons.

(c) $HSO_4^-$

An amphiprotic species can both donate and accept a proton. $HSO_4^-$ can lose $H^+$ (giving $SO_4^{2-}$) or gain $H^+$ (giving $H_2SO_4$).

(a) Not a buffer solution with pH<7

A buffer needs a weak acid and its conjugate base (or weak base and its conjugate acid). HCl is a strong acid and NaCl is a neutral salt, so it is not a buffer. The strong acid makes pH $<7$ (about 0).

(b) Lattice energy

The stability of an ionic compound depends on the energy released when the ions pack into the crystal, which is the lattice energy.

(d) Temperature is increased

$K_w$ depends only on temperature. Dissociation of water is endothermic, so $K_w$ increases as temperature rises.

(b) Cu

Malachite is $CuCO_3\cdot Cu(OH)_2$, an ore of copper.

(d) $NH_3$ and $CO_2$

In the Solvay process $CO_2$ from heating $NaHCO_3$ and limestone is reused, and $NH_3$ is recovered from $NH_4Cl$ by heating with $Ca(OH)_2$ and used again.

(a) $NO_2$

$NO_2$ is a reddish-brown gas. $NO$, $N_2O$ and $N_2O_4$ are colourless.

(b) NaCl + $NaClO_3$

With hot concentrated alkali: $3Cl_2+6NaOH\to5NaCl+NaClO_3+3H_2O$. (Cold dilute NaOH gives NaCl + NaClO.)

(b) $AlH_3$

Covalent character is greatest when the electronegativity difference is smallest and the cation is small and highly charged (Fajans' rules). The Al-H difference ($\approx0.6$) is smaller than Al-Cl ($\approx1.5$), so $AlH_3$ is the most covalent. $MgI_2$ and NaI are more ionic.

(b) Vacuum distillation

Glycerin decomposes at its normal boiling point, so it is distilled under reduced pressure at a lower temperature.

(d) 75

25 cc of diluted solution needs $17\times0.1 = 1.7$ meq HCl, so the whole 250 cc holds 17 meq of $Na_2CO_3$. Equivalent weight of $Na_2CO_3 = 106/2 = 53$, so mass $= 17\times10^{-3}\times53 = 0.901$ g.

$\% = \frac{0.901}{1.2}\times100 \approx 75\%$

(d) $E_2F_6$

Element in oxide: $31.04\%$, so equivalent weight $= \dfrac{31.04}{68.96}\times8 = 3.6$. Approximate atomic weight (Dulong-Petit) $= 6.4/0.59 \approx 10.85$, so valency $= 10.85/3.6 \approx 3$ and exact atomic weight $= 3\times3.6 = 10.8$ (boron).

Molar mass of the fluoride $= 0.605\times\dfrac{22400}{100} = 135.5$. $BF_3 = 67.8$, so the molecule is $B_2F_6 = 135.6$, i.e. $E_2F_6$.

(c) S is precipitated

Roasting pyrite gives $SO_2$. It oxidises $H_2S$ (hydride of sulphur): $SO_2 + 2H_2S \to 3S\downarrow + 2H_2O$.

(c) Ethane, ethene & ethyne

Kolbe electrolysis of sodium acetate gives ethane ($CH_3-CH_3$), sodium succinate gives ethene ($CH_2=CH_2$), and sodium maleate (or fumarate) gives ethyne ($HC\equiv CH$).