With a mighty hand – This refers to the plague of pestilence… How do we know that the expression, “A mighty hand,” is a reference to the fifth plague, Dever or Pestilence?

The answer is to be found in how we see the overall count of the plagues. If we look carefully at the way in which the ten plagues are described in the Torah, one will see that they can be divided into two groups of five plagues each. In the first grouping of five plagues, Pharaoh hardens his own heart but in the second grouping of five plagues, God causes Pharaoh’s heart to be hardened. Pestilence is the fifth plague. Therefore the Egyptians experienced this plague, they had experienced an entire “hand” of plagues (especially when we consider that the third plague is referred to as “The finger of God”). The expression “the Hand of God” is also used in reference to the tenth plague. ”