"How is absorbance and transmittance related to pH?"
To figure this out, you need to make a two standard curves (graphs) using something like Microsoft Excel. Using the absorbance values for the solutions of known pH, plot absorbance versus pH. Then do the same for transmittance versus pH.
In Excel, you can then use the linear regression function to get an equation in the form of y = mx + b. Then, using the spectrometer values you collected for the unknown solutions, you can solve for the pH.
For instance, you plot pH on the x-axis and transmittance on the y-axis. Unknown solution #1 has an transmittance of 10. You can then plug it in to the equation for y and solve for x (bear in mind you will have the slope, m, and also the constant b once you do the linear regression). For the purpose of simplicity, we'll say m = 1 and b = 5
y = 1x + 5
10 = 1x + 5
5 = 1x
x = 5 = pH