What is the net charge of histidine at pH = pKa2?(pKa1 = 1.8, pKa2 = 6.0, pKa3 = 9.3

What causes temperature extremes in the desert

If animal agriculture was eliminated, how much grain would become available for human consumption?

Tom wants to conduct a scientific study but he needs to finish by the end of the school year. what practical way could tom work around this limitation and be successful in his study?