(a) Explain briefly the observations in each of the following processes:
(i) when carbon (IV) oxide is bubbled through lime water, It turns milky but the milkiness disappears when the gas is bubbled for a long time.
(ii) A precipitate of calcium hydroxide is insoluble in excess sodium hydroxide solution whereas that of lead (ii) hydroxide is soluble
(b)(i) What is a primary standard solution?
(ii) Calculate the mass of sodium trioxocarbonate (V) required to prepare 250 cm\(^3\) of 0.15gmoldm\(^3\) solution hydroxide is soluble. [Na = 23.0; O = 16.0; C = 12.0]
(c) Name one gas that can be collected by
(i) upward displacement of air
(ii) downward displacement of air
Explanation
(a)(i) Lime water turns milky with CO\(_2\) because insoluble CaCO\(_3\) / CacO\(_{3(s)}\) is formed.
Milkiness disappears when excess CO\(_2\) reacts with CaCO\(_3\) in water medium forming the soluble Ca(HCO\(_3\)\(_2\))/Ca(HCO\(_3\))\(_2\)
ii) Calcium hydroxide is not amphoteric. Does not react with an alkali NaOH whereas lead (II) hydroxide is amphoteric, so reacts with excess NaOH
(b)(i) Primary standard solution is one whose concentration is known and can be used to standardize another solution OR Primary standard solution is a solution of known concentration prepared from pure / non - deliquescent / non hygroscopic substances.
(ii) Molar mass Na\(_2\)CO\(_3\) = 106gmol\(^{-1}\)
Molar mass Na\(_2\)CO\(_3\) = C x M x V
= 0.15 x 106 x 0.25
= 0.15 x 106 x 0.25
= 3.98g