a sample of krypton had a volume of 10L at 1atm pressure and 273K. how many krypton moles were present?

Use the ideal gas law to calculate the number of moles:

PV = n RT
n = PV/RT = 1 atm (10 L) / (0.0821 Latm/molK) (273K)
n = 0.446 mol

The other approach is to recognize that your temperature and pressure are the conditions of STP (standard temperature and pressure). At STP, 1 mole of any gas occupies a volume of 22.4 L. So,

moles Kr = 10 L X (1 mol/22.4L) = 0.446 mol

using the equation PV=nRT and subititute the vaules you've been given.

10L=0.01m^3 1atm pressure=100kPa

0.01m^3 x 100kPa= n x 8.31 x 273K
rearrange to get n

0.01m^3 x 100kPa / 8.31 x 273K =n

and n should =0.441moles(3.s.f)