Q48: This is asking about Michaelis-Menten kinetics.
There are two sections of the Michaelis-Menten graph: when the enzyme is unsaturated and when the enzyme is saturated.
https://lh3.googleuserco...C_8dmTGCAPjrzITeYrT44mFIIt is MUCH easier to measure the rate constant of the catalyst (kcat) when [S] is saturated, because you're in that flat region, where kcat stops changing.
When [S] is too low, then you need to find the slope of the line, and relate it to kcat/KM, meaning you also need to know KM. Not fun. So, dump a ton of substrate in until the rate of product formation doesn't change, and you know you're in the right spot!
Q51: Be careful! At pH = 7, histidine is UNCHARGED (neutral).
Here are the total charged species:
Two glutamic acids and one aspartic acid (-3).
One lysine and one arginine (+2).
Overall: -1.
Q57: I think you're thinking about it backwards. Here, they're asking you to start with the diluted concentration, and then figure out the more concentrated stuff. That's why we're multiplying!