What is a standard solution? What mass of sodium carbonate is needed to prepare 500 cm 3 of 0.2 moldm -3 solution? How much water is needed to dilute the solution in a (ii) to 0.04 moldm -3 ?

1(a)

Molar mass of $\mathrm{Na_2CO_3}$:
$2\times23 + 12 + 3\times16 = 46 + 12 + 48 = 106\,\mathrm{g/mol}$

Step 1:
$\text{Mass} = n \times M$
$= 0.4\,mol \times 106\,\mathrm{g/mol}$
$= 42.4\,\mathrm{g}$

42.4 g

1(b)

Step 1:
$c = \frac{n}{V}$
$V = \frac{n}{c}$
$= \frac{0.4\,mol}{0.3\,mol/L}$
$= 1.33\,\mathrm{L}$

1.33 L

2(a)

Step 1:
$n = c \times V$
$= 0.2\,mol/L \times 0.5\,\mathrm{L}$
$= 0.1\,\mathrm{mol}$

Step 2:
$\text{Mass} = n \times M$
$= 0.1\,mol \times 106\,\mathrm{g/mol}$
$= 10.6\,\mathrm{g}$

10.6 g

2(b)

Current: $0.5\,\mathrm{L}$ of $0.2\,\mathrm{M}$, $n=0.1\,\mathrm{mol}$

Step 1:
New $c=0.1\,\mathrm{M}$
$V' = \frac{n}{c'} = \frac{0.1\,mol}{0.1\,mol/L}$
$= 1\,\mathrm{L}$

Step 2:
Water added $= V' - V = 1\,L - 0.5\,\mathrm{L}$
$= 0.5\,L = 500\,\mathrm{mL}$

500 mL

3

Moles $\mathrm{AgNO_3}$:
$n_{Ag^+} = 0.1\,mol/L \times 0.025\,L = 0.0025\,\mathrm{mol}$

Moles $\mathrm{NaCl}$:
$n_{Cl^-} = 0.2\,mol/L \times 0.020\,L = 0.004\,\mathrm{mol}$

$\mathrm{NaCl}$ solution is in excess (more $\mathrm{Cl^-}$).

Step 1:
Excess $\mathrm{Cl^-} = 0.004\,mol - 0.0025\,\mathrm{mol}$
$= 0.0015\,\mathrm{mol}$

\mathrm{NaCl solution; 0.0015 mol}

4

(d)
Body-centered cubic structure is used to calculate lattice energy for metals like Fe, Cr, etc.

(e)
Lattice energy is the energy released when one mole of an ionic compound in gaseous state forms a solid crystal lattice.
Lattice energy: energy released on forming 1 mol ionic solid from gaseous ions