Use ideal gas equation
pV = n RT => p = nRT / V
T = 25 + 273.15 K = 298.15 K
V = 684 ml = 0.684 l
R = 0.0821 atm * l / (K*mol)
n = mass / molar mass =
molar mass of CO2 = 12 g/mol + 2*16 g/mol = 44 g/mol
n = 6.43 g / 44 g/mol = 0.14614 mol
=> p = 0.14614 mol * 0.0821 atm*l /(K*mol) * 298.15K / 0.684 l = 5.23 atm
Answer: p = 5.23 atm