The answer is 1.6 atm. Let's first calculate the mole fraction of gas Y. The mole fraction (x) is: x = n1/n, where n1 is a number of moles of an individual gas in a mixture and n is total moles of the gas mixture. We know that n1 of gas Y is 6.0 mol (n1 = 6.0 mol) and that there are in total 8. mol of the gas mixture (n = 2.0 + 6.0 = 8.0 mol). Now calculate the mole fraction of gas Y. x = 6.0/8.0 = 0.75. Now, let's use the mole fraction of gas Y (x) and the total pressure (P) to calculate the partial pressure of gas Y (P1): x = P1/P. P1 = x * P. If x = 0.75 and P = 2.1 atm, then the partial pressure of gas Y is: P1 = 0.75 * 2.1 atm = 1.6 atm.