400×10=4,000
4000
400 multiplied by 10 is 4,000.
The equation needed is: V=fl Where V is the speed, f is the frequency and l is the wavelength. (the poxy thing won't let me enter a lamba sign) The key bit of information is "in a vacuum"; all waves in a vacuum travel at the same speed, 3x108ms-1. This is more commenly refered to as the speed of light, C. Therefore, f = V / l= C / l = 3x108 / 400x10-9 = 7.5x1014 Hz (Not in standard form that's: 750,000,000,000,000 Hz) (The '/' means divided by)
To do this, you need to know the molecular weight of the element you're dealing with, by adding up the atomic weights of the elements involved (found on any periodic table). The molecular weight is the mass in grams of the compound in one mole - this will provide you with a conversion factor. So take the measurement in grams and divide it by the molecular weight to convert to moles. Really what you're doing is multiplying the number by 1 mole, and dividing it by the equivalent of one mole, the molecular weight. That's the thought process behind unit analysis and how you get your "units to cancel".In this case, the answer is about 2 grams NaOH.