4.
$$10\,mL$$ of water requires $$1.47\,mg$$ of $${K_2}C{r_2}{O_7}$$ $$\left( {M.\,wt. = 294} \right)$$ for oxidation of dissolved organic matter. $$C.O.D.$$ is
$$10\,mL$$ water $$ = 1.47 \times {10^{ - 3}}\,g\,$$ of $${K_2}C{r_2}{O_7}$$
$${10^6}\,mL$$ of water $$ \equiv 147g$$ of $${K_2}C{r_2}{O_7}$$
No. of gm equivalents of dichromate reacts is equal to the no. of gm equivalents of oxygen required.
$$\therefore 49\,g$$ of $${K_2}C{r_2}{O_7}$$ requires $${O_2} = 8\,g$$
$$\left[ {gm.\,eqv.\,\,{\text{of}}\,\,{K_2}C{r_2}{O_7} = \frac{{294}}{6} = 49,\,gm\,eqv.\,\,{\text{of}}\,\,{\text{oxygen}} = 8} \right]$$
$$\therefore 147\,g$$ of $${K_2}C{r_2}{O_7}$$ will require $${O_2} = \frac{{8 \times 147}}{{49}} = 24\,g$$ $$\left[ {24\,g = 24\,mg/L = 24\,ppm} \right]$$
5.
Which of the following statements about the depletion of ozone layer is correct ?
A.
The problem of ozone depletion is less serious at poles because $$N{O_2}$$ solidifies and is not available for consuming $$CI{O^ \bullet }$$ radicals.
B.
The problem of ozone depletion is more
serious at poles because ice crystals in the clouds over poles act as catalyst for photochemical reactions involving the decomposition of ozone of $$C{l^ \bullet }$$ and $$CI{O^ \bullet }$$ radicals.
C.
Freons, chlorofluorocarbons, are inert. Chemically, they do not react with ozone in
stratosphere.
D.
Oxides of nitrogen also do not react with ozone in stratosphere.
Answer :
Freons, chlorofluorocarbons, are inert. Chemically, they do not react with ozone in
stratosphere.
Freons and chlorofluorocarbon find their way to stratosphere through without being destroyed in the troposphere due to their low reactivity. In the stratosphere the $$Cl$$ and $$Br$$ atoms are liberated from the parent compounds by the action of ultraviolet light
e.g.
$$\eqalign{
& C{F_2}C{l_2} + h\nu \to C{F_2}Cl + Cl \cr
& CFC{l_3} + h\nu \to CFC{l_2} + Cl \cr} $$
The $$Cl$$ atoms can destroy ozone molecules through a variety of catalytic cycles.
6.
The two strong acids present in the acid rain are
Carbon dioxide, methane, water vapour, nitrous oxide, $$CFCs$$ and ozone are greenhouse gases.
10.
Consider the following statements :
(i) Zirconium-Alizarins dye is used for testing fluoride ions in water.
(ii) Ozone layer is present in mesosphere.
(iii) The poisonous gas present in the exhaust fumes of automobiles is $$CO.$$
(iv) Taj Mahal is affected by $$C{O_2}$$ gas.
The correct statements are