Page 84 - Chemistry ICSE Class X

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Colour of litmus paper at different
pH values
dŽŽƚŚƉĂƐƚĞƐ ĂƌĞ ƐůŝŐŚƚůLJ ďĂƐŝĐ͘ ^Ž͕
ƚŚĞLJ ŶĞƵƚƌĂůŝƐĞ ƚŚĞ ĞdžĐĞƐƐ ĂĐŝĚ
ĂƌŽƵŶĚ ƚĞĞƚŚ͘
ͻ acidity for the given acids is
,ŽŶĞLJďĞĞ ƐƟŶŐ ĂŶĚ EĞƩůĞ ƐƟŶŐ
ƌĞůĞĂƐĞ ŵĞƚŚĂŶŽŝĐ ĂĐŝĚ ;ĨŽƌŵŝĐ
ĂĐŝĚͿ͘ dŚŝƐ ĐĂƵƐĞƐ ƉĂŝŶ ĂŶĚ ŝƌƌŝƚĂƟŽŶ͘
ZƵďďŝŶŐ Ă ŵŝůĚ ďĂƐĞ ƐƵĐŚ ĂƐ ďĂŬŝŶŐ
ƐŽĚĂ ;ƐŽĚŝƵŵ ŚLJĚƌŽŐĞŶĐĂƌďŽŶĂƚĞͿ
ŽǀĞƌ ƚŚĞ ĂīĞĐƚĞĚ ĂƌĞĂ ŐŝǀĞƐ ƌĞůŝĞĨ͘ Solution: (i)
ICSE Chemistry – 10
This excess acid causes irritation. This is commonly called acidity.
To have relief from acidity, we generally take antacids. Antacids
neutralise the excess acid. Commonly used antacids contain
aluminium hydroxide or magnesium hydroxide.
ƒ pH of blood is maintained within the range of 7.36 to 7.42. This is due
to the buffer action of bicarbonate-carbonic acid system.
ƒ Enzymes function effectively only at certain pH, e.g. trypsin acts best
at an alkaline pH.
ƒ The control of pH values is very important from the point of view of
CPCN[VKECN EJGOKUVT[ 6JKU RGTOKVU UGNGEVKXG FGVGEVKQP KFGPVKſECVKQP
and/or isolation of certain substances in the presence of others.
ƒ The pH change as the cause of tooth decay: The tooth enamel,
made up of calcium phosphate, is the hardest material in our body.
It does not dissolve in water. It gets corroded at pH below 5.5, i.e.,
in moderately acidic conditions. The bacteria produce acid in the
mouth during degradation of sugar and food particles (left behind
after meal). This acid corrodes the teeth. To prevent tooth decay, one
should brush teeth properly after each meal. Another way to prevent
tooth decay is to avoid eating sugary foods.
ƒ Control of pH of the soil: 2NCPVU CNUQ PGGF C URGEKſE R* TCPIG HQT
proper growth. The soil may be acidic, basic or neutral depending
+
–
upon the relative concentration of H and OH . The pH of any soil
can be determined by using pH paper.
–4
EXAMPLE 3.1 Calculate the pH of a solution which is 1 × 10 mol/L in HCl.
Solution: HCl in dilute solution can be assumed to be completely ionised.
–4
+
So, [H ] = [HCl] = 1 × 10 mol L –1
+
–4
Then, pH = – log [H ] = – log (1 × 10 ) = –(– 4) log 10
pH = 4
–4
Thus, a 1 × 10 mol/L solution of HCl has a pH of 4.
EXAMPLE 3.2 The pH values of three acids A, B and C having equal molar
concentrations are 5.0, 2.8 and 3.5 at 298 K. Arrange these acids in order of their
increasing acid strength.
Solution: At 298 K,
pH of A = 5.0
pH of B = 2.8
pH of C = 3.5
+
An acid having higher acidity will have higher concentration of H ion in the
solution and, therefore, will have lower pH. Therefore, the order of increasing
A < C < B
EXAMPLE 3.3 Calculate the pH of the solution when hydrogen ion concentration
is: (i) 1.0 × 10 M (ii) 1.0 × 10 M
–9
–7
+ –7 –1
3
[H O ] = 1.0 × 10 mol L
+
So, pH = – log [H O ]
3
–7
= – log (1.0 × 10 ) = 7.0
–9
+
(ii) [H O ] = 1.0 × 10 mol L –1
3
+
So, pH = – log [H O ]
3
–9
= – log (1.0 × 10 ) = 9.0