Question

3. (10 pts) What does the following statement represent? pH=- log (1.2 M) 4. (10 pts) How does the pH probe determine the pH of a 6.0M solution of HNO,?

Answer 1

3. We know that pH is the negative log of the molar hydrogen ion concentration.

Thus if a HA acid molecule dissolve in water and dissociates as: HA = H+ + A- . Then pH can be determined by the following formula.

pH = —log [H+]

Given that pH = —log [1.2M]

So, from here we can conclude that the molar concentration of hydrogen ion after dissociation is 1.2M. and from this we can determine the pH of the solution also. Thus pH = -log 1.2 = —0.079.

Here we have negetive pH. any acid that yields a concentration of hydrogen ions with a molarity greater than 1 will be calculated to have a negative pH (here we have concentration of 1.2M). This is because glass pH electrodes suffer from a defect called 'acid error' which causes them to measure a higher pH than the real pH.

4. pH of a solution can be determined by negetive logarithm of the molar concentration of hydrogen ion. i.e., pH = —log [H+]

Now we have to find pH of 6.0 M HNO3 solution. Let we assume that HNO3 fully dissociates in the solution as: HNO3 = H+ + NO3-

Here 1mol HNO3 produce 1mol H+ and 1mol No3- . Thus 6M HNO3 produce 6M of H+ & 6M of NO3-.

Now pH = - log [H+]

pH = - log 6

=> pH = -0.778

So, pH of 6M HNO3 solution is -0.778.