What is the pH of 1M NaOH, 1M HCl solution, saliva before meal, saliva after meal and lemon juice? Give the pH with complete solutions.

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Solution

pH is defined as the negative log of Hydrogen ion concentration.
pH = -log[H⁺]
a similar relation is there to express the OH^- ion concentration for bases. It is written as
pOH = -log[OH^-]
pH and pOH are related by the following expression
pH + pOH = 14
The pH of 1M HCl would therefore be
= - log [H⁺]
= - log [1] = 0
Therefore pH of 1 M HCl solution is 0. Similarly pOH of 1M HCl will be
= -log[OH^-]
= - log [1] = 0
Hence the pH of 1M NaOH will be
= 14 - pOH
= 14 - 0 = 14
The pH of saliva is 6.8. By how much this pH value will change depends upon the kind of meal taken by the person. If a meal is taken which has more acidic content, then pH of saliva will drop. On the other hand, if a meal which has more basic content is taken, the pH will increase. Citric acid is a weak organic acid, hence its pH will be close to 6.

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