amu) = 80 amu. Oxygen’s percent, by mass, is × 100% or 60%. (C)
is correct.
- T, F Divide and conquer! Look at the first statement. Is it true or false?
Of course it’s true. Chlorine is an element. What about statement II?
It’s false. Remember that chlorine exists in diatomic form at room
temperature and atmospheric pressure.
- T, T, CE
Divide and conquer. Consider statement I. One mole of
HBr contains the same number of molecules as one
mole of NO 2 —6.02 × 10^23 molecules—but this doesn’t
necessarily mean that 6.02 × 10^23 HBr molecules weigh
the same as 6.02 × 10^23 NO 2 molecules. Do ten paper
clips weigh the same as ten elephants? Certainly not.
Use the periodic table. One molecule of HBr has a mass
of about 1 amu + 80 amu, or 81 amu. One molecule of
NO 2 has a mass of about 14 amu + 2(16 amu), or 46
amu, so one molecule of HBr has greater mass than one
molecule of NO 2 . And one mole of HBr would thus
have greater mass than one mole of NO 2 . Statement I is
true.
What about statement II? Well, we’ve already
determined that it’s also true.
Now does this sentence make sense? “One mole of HBr
has greater mass than one mole of NO 2 because the
mass of a molecule of HBr is greater than the mass of a
molecule of NO 2 .” It sure does, so fill in the oval
marked CE.
- B Remember that to calculate the formula weight of Ca(NO 3 ) 2 , all
you need to do is add up the weights of its constituent atoms. Calcium’s
