4 Marks Questions

Question 14 Marks

Which mechanisms helps in the transportation of water in a plant?

Answer

In an upward direction, osmosis helps in the transportation of water in a plant.

Question 24 Marks

Read the following text carefully and answer the questions that follow:
Many chemical and biological processes depend on osmosis, the selective passage of solvent molecules through the porous membrane from a dilute solution to a more concentrated one. The osmotic pressure $\pi$ depends on molar concentration of the solution ($\pi$ = CRT). If two solutions are of equal solute concentration and, hence, have the same osmotic pressure, they are said to be isotonic. If two solutions are of unequal osmotic pressures, the more concentrated solution is said to be hypertonic and the more diluted solution is described as hypotonic. Osmosis is the major mechanism, for transporting water upward in the plants. Transpiration is the leaves supports the transport mechanism of water. The osmotic pressure of seawater is about 30 atm; this is the pressure that must be applied to the seawater (separated from pure water using a semi-permeable membrane) to get drinking water.
i. What will happen if a plant cell kept in a hypertonic solution?
ii. Blood cells are isotonic with 0.9% sodium chloride solution. What happens if we place blood cells in a solution containing in 1.2% sodium chloride solution?
iii. What happens when the external pressure applied becomes more than the osmotic pressure of solution?

Answer

i. A plant cell gets shrink when it is kept in a hypertonic solution.
ii. 1.2% sodium chloride solution is hypertonic with respect to 0.9% sodium chloride solution or blood cells. When blood cells are placed in this solution, water flows out of the cells and they shrink due to loss of water by osmosis.
iii. When the external pressure applied becomes more than the osmotic pressure of the solution, then the solvent molecules from the solution pass through the semipermeable membrane to the solvent side. This process is called reverse osmosis.

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Question 34 Marks

$MnO$ is basic whereas $Mn_2O_7$ is acidic in nature. Give reason.

Answer

When a metal is in a high oxidation state, its oxide is acidic and when a metal is in a low oxidation state its oxide is basic.

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Question 44 Marks

Read the following text carefully and answer the questions that follow:
Transition metal oxides are generally formed by the reaction of metals with oxygen at high temperatures. The highest oxidation number in the oxides coincides with the group number. In vanadium, there is a gradual change from the basic $V_2O_3$ to less basic $V_2O_4$ and to amphoteric $V_2O_5. V_2O_4$ dissolves in acids to give $VO^{2+}$ salts. Potassium dichromate is a very important chemical used in the leather industry and as an oxidant for the preparation of many azo compounds. Dichromates are generally prepared from chromate. Sodium dichromate is more soluble than potassium dichromate. The latter is, therefore, prepared by treating the solution of sodium dichromate with potassium chloride. Sodium and potassium dichromates are strong oxidising agents; sodium salt has a greater solubility in water and is extensively used as an oxidising agent in organic chemistry. Potassium dichromate is used as a primary standard in volumetric analysis.
$i.$ Which of the $3d$ series of the transition metals exhibits the largest number of oxidation and why?
$ii.$ A transition metal exhibits highest oxidation state ih oxides and fluorides. Give reason.
$iii.$ How would you account for the increasing oxidising power in the series: $VO _2^{+}< Cr _2 O _7^{2-}< MnO _4^{-}$ ?

Answer

$i.$ Manganese $(Z = 25) $ shows maximum number of $O.S.$ This is because its outer $EC$ is $3d^{5}4s^{2}$. As $3d$ and $4s$ are close in energy, it has maximum number of $e-1s$ to loose or share. Hence, it shows $O.S.$ from $+2$ to $+7$ which is the maximum number.Page $12$ of $14$
$ii.$ A transition metal exhibits higher oxidation states in oxides and fluorides because oxygen and fluorine are highly electronegative elements, small in size and strongest oxidising agents.
$iii.$ This is due to the increasing stability of the lower species to which they are reduced.

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