Question

A beer has a pH of 4.30. What is the value of [ $ H_3O^+ $ ]?

Answer 1

Hint: pH scale is a scale whose major function is to determine how acidic or basic or neutral a given compound is. pH actually refers to the ‘potential of hydrogen’. It is the measure of the relative amount of free hydrogen in water.
 $ pH = - \log \left[ {{H^ + }} \right] $

Complete answer:
We know that pH refers to the relative amount of hydrogen in water. It tells how acidic or basic a given compound is. Here it has been given that the pH of beer is 4.3. we have to find the value of [ $ H_3O^+ $ ].
To find this we will use the above mentioned formula :
We know that pH is negative of logarithm of $H^+$ ion or hydronium ion.
Here it is given that :
 $ pH = - \log \left[ {{H^ + }} \right] $
It can also be written as :
 $ pH = - \log \left[ {{H_3}{O^ + }} \right] $
Here pH has been given the value 4.3, hence
Or, $ 4.3 = - \log \left[ {{H_3}{O^ + }} \right] $
Or, $ \log \left[ {{H_3}{O^ + }} \right] = - 4.3 $
Now taking antilog in the equation we get:
 $ \left[ {{H_3}{O^ + }} \right] = anti\log \left( { - 4.3} \right) $
 $ \left[ {{H_3}{O^ + }} \right] = anti\log \left( {0.70} \right) \times {10^{ - 5}} $
 $ \left[ {{H_3}{O^ + }} \right] = 5.012 \times {10^{ - 5}}g.ions.litr{e^{ - 1}} $
Hence the value of [ $ H_3O^+ $ ] is $ 5.012 \times {10^{ - 5}}g.ions.litr{e^{ - 1}} $ .

Additional Information:
Just like ph there is another term called pOH. It is defined as the negative logarithm of hydroxide ion. We say that neutrality of pH is 7 but that is not ideal. Neutrality is defined only when $ [{H^ + }] $ = $ [O{H^ - }] $ .pH is used to make pH indicators and they occur in many plants like hibiscus, red cabbage , grapes, etc. it is even present in our stomach and helps to maintain neutrality otherwise it causes acidity.

Note:
Remember that pH is the negative logarithm of hydronium ion, not the positive value of logarithm.
The pH being 4.3 is less than 7, it tells us that the solution is acidic in nature.
Any value on pH scale less than 7 is considered acidic and any value more than 7 is basic.