What does the little 2 in 2CO2 mean?

Calculate the molarity of 48.0 ml of 6.00 m h2so4 diluted to 0.250 l .

What is the mass of sodium in 3 moles of sodium chloride

Suppose you mix one mole of sulfuric acid (h2so4) with 1 mole of sodium hydroxide(naoh). why does the ph of the solution remain below 7? ( explain so i can get better understanding! )