Alight wave travels at a speed of 3.0 × 108 meters/second. if the wavelength is 7.0 × 10-7 meters, what is the frequency of the wave?

c = f*L 

If you cant remember this just consider the units of each (denoted [c] is units of c): 

[c] = meters/second, [f] = 1/seconds or Hertz, [L] = meters 

So [c] = [f*L], an easy way to remember it. 

So to get the frequency, just solve for f: 

f = c/L = (3.00 x 10^8)/(5.93 x 10^-7) m/s = 5.05902192 × 10^14 m/s ~ 5.06 x 10^14 m/s 

in the given question, the partial pressure is calculated by using the ideal gas equation, that is, pv = nrt

here, p is pressure, v is volume = 25 l, n is number of moles = 0.0104 moles, r is gas constant = -0.0821 l.atm / mol.k

t is temperature = 273 k

p = nrt / v

p = (0.0104 mol × 0.0821 l.atm / mol.k × 273 k) / 25 l

= 0.0093 atm