What is the ph difference of two samples if the concentration of [h+] ions is 1000-fold less in the second sample?
a. the ph of the first sample would be 3 ph units lower than the second sample.
b. the ph of the first sample would be 2 ph units higher than the second sample.
c. the ph of the first sample would be 3 ph units higher than the second sample.
d. the ph of the first sample would be 2 ph units lower than the second sample.

The correct answer is a. This is because the pH of a solution is defined as -log10(concentration of H+ ions). An inverse logarithmic scale such as this means that a solution with a lower concentration of H+ ions will have a higher pH than one with a higher concentration. Therefore we know that the pH of the second sample will be higher than the first.

Since the logarithmic scale has the base 10, a change by 1 on the scale is a consequence of multiplication/division of the H+ concentration by a factor of 10. As the scale is inverse, this means that a decrease of concentration by factor 1000 is equivalent to increasing the pH by (1000/10) = 3.