Or HaChaim on Numbers 21:7/אור החיים על במדבר כ״א:ז׳

1 התפלל אל השם, "pray to the merciful G'd, etc." In this instance the meaning of the word התפלל is equivalent to a call for forgiveness of the sin. There is a difference in the accessibility of forgiveness if one prays for it before the guilty party has experienced afflictions or if one has waited until after he has experienced afflictions. Once the guilty party has already become the victim of the destructive forces he himself has created and which have come home to roost, he needs to marshall some merit in order for these destructive forces to be called off. Shabbat 32 has this to say on the subject: "A person should make a point of asking for mercy before he falls sick because once he has fallen sick they say to him: 'produce some merit and you will be freed from the sickness.'" This is why the people of the generation of the desert (also known as the דור דעה, the generation blessed with knowledge) were astute enough to ask Moses to pray before asking him to remove the destructive serpents. They wanted Moses to invoke his own merits on their behalf.

2את הנחש, the serpent. They wanted that G'd should remove even the relatively minor problem, the deadly bite of the snake that attacked only the bodies. If they had mentioned the שרף or the נחש השרף it would have sounded as if they were only concerned about not having their souls destroyed but would have been content to die a merely physical death, i.e. a normal death. By mentioning only the נחש, the people made it plain that they did not want to die a natural death of the body, and if so, they certainly did not want their souls to die. They were also careful to speak of the נחש in the singular to indicate that they did not want a single snake to remain.