Question

The titration curve for weak acid and weak base has no sharp changes. For this titration no suitable indicator is present.
Reason
Indicator should change its colour sharply for indication of reaction to be complete so its PH-range lies perfectly within sharp change to avoid experimental error.
A.Both assertion and reason are correct and reason is the correct explanation for assertion
B.Both assertion and reason are correct but reason is not the explanation of assertion
C.Assertion is correct but reason is incorrect
D.Assertion is incorrect but reason is correct.

Answer 1

Hint: You must recall how and why an indicator changes its colour while titrating. Weak acid and weak bases reaction is very slow in process.

Complete step by step answer:
Before going to the question let's understand the terminology used here.
Indicators-are either organic weak acids or weak bases with characteristics of having different colours in the ionized and unionized form. For example phenolphthalein is a weak acid whose ionized form gives a pink color and unionized form is colourless. It helps in the identification of acids and bases by giving a characteristics colour over a pH range. For different pH ranges different indicators are used.
Weak acids and weak bases- Acids and bases that don't dissociate completely. For example acetic acid is a weak acid and ammonium hydroxide is a weak base.
Now come to the assertion part – The reaction between a weak acid and weak base is very very slow in nature that no steep rise occurs near the equivalence point nor the rise of pH encompasses an interval equal to pH transition range.Hence no indicator can be used to detect the end point. If we try to do it then the result will be full of error.
Now come to the reason part- So the given reason is the characteristics property of indicator. Why do we use an indicator? Because they give us a characteristic colour based upon which we differentiate whether the given compound is acidic or basic.
And the change should be so quick because even an extra drop of solution can cause error in the experiment.
Hence, both the assertion and reason are correct and reason is the well explained explanation of assertion.

So, the correct answer is option A.

Note:
Equivalence point is a point where the number of moles of an acid becomes equal to the number of moles of base.