Question

At $25^{o}C, \, pH$ range of phenolphthalein is $8-10.$ At $100^{o}C,$ $\text{pH}$ range of phenolphthalein would be -

• A. $\text{pH}$ range remain unaffected by the temperature
• B. $\text{pH}$ range is altered to $7$ - $9$
• C. $\text{pH}$ range is altered to $\text{7}$ - $\text{11}$
• D. $\text{pH}$ range is altered to $\text{8}$ - $\text{11}$

Answer 1

Answer: (B)

The $\text{pH}$ range of the indicator is $pK_{I n}\pm1.$ On increase in temperature $pK_{I n}$ decreases therefore $\text{pH}$ range of indicator decreases.