A compound is 57.1% oxygen and the remainder carbon. using oxygen as the standard (16 amu) and assuming a 1:1 atom ratio, calculate the relative mass of a carbon atom.

Let's start with the amount given in percent. Let our basis be 100 grams of compound. So, that means that in this amount, 57.1 g is oxygen and 100-57.1=42.9 g is carbon. Since there is 1:1 atom ratio, it also means that moles oxygen = moles carbon.

Moles = Mass/Relative Mass

Let x be the relative mass of oxygen

57.1/16 = 42.9/x

Solving for x,

x = 12.02 amu