Chapter 4 - Inverse, Exponential, and Logarithmic Functions - 4.4 Evaluation Logarithms and the Change-of-Base Theorem - 4.4 Exercises - Page 457: 31

$ 8.4$

Apply logarithmic property : $\log m+ \log n=\log (mn)$ We can write the equation as: $ pH=-\log [3.9 \times 10^{-9}] =-[\log 3.9 + \log 10^{-9}]$ Now, apply logarithmic property : $\log a^ b=b \log a$ Therefore, our answer is: $-[\log 3.9 - 9 \log 10]=-[0.591-9]= 8.4$