Lowest to Highest pH

We are given that the $\text{pH}$ of a substance could be determined by $\text{pH} = -\log{[H^+]}$ and the maximum possible $\text{pH}$ of a substance is $14.$ So setting $14 = -\log{[H^+]} \implies [H^+] = \dfrac{1}{10^{14}}$ . Since the formula for $\text{pH}$ is a negative logarithm, we can see that a higher $\text{pH}$ would imply a lower $[H^+]$ , and since the greater of $a$ or $b$ would give the lowest $[H^+]$ , and $a+b = 15$ , we get that $b = 14 \implies a = 1.$

From this, we see that $\text{pH} = -\log{\dfrac{1}{10}} \implies \log{10} = 1.$

Thus, the lowest possible $\text{pH}$ here is $\boxed{1}$