The ionic substance strontium oxide, SrO, forms from the reaction of strontium metal with molecular oxygen. The arrangement of the ions in solid SrO is analogous to that in solid NaCl:

(c) The density of SrO is 5.10 g>cm3. Given your answer to part (b), how many formula units of SrO are contained in the cube shown here?

Video transcript

Hi everyone here we have a question telling us that a portion of the potassium bromide lattice is shown below the density of potassium bromide is 2.75 g per centimeters cube. And our goal is to calculate the number of formula units of potassium bromide contained in the cube shown. And we're going to assume that the edge and corners of the cube are in the center of the atoms. So from a table of values we know that potassium has a radius of 1.52 Angstrom bromide has a radius of 1.82 Angstrom assuming that the corner ends in the center of the corner atom. This means we only use one of the radius of the corner atoms and we use twice the radius for the inside atoms. So the top edge is is browning, potassium browning. So browning plus two potassium plus browning. So 1. angstrom Plus two times 1.52 Angstrom Plus 1.82 Angstrom Equals 6.68 Angstrom. So now we're going to calculate the volume. The volume equals s cubed, So volume equals 6.68 Angstrom times To the negative 10th m divided by one angstrom times one centimeter Over 10 to the -2 m cubed And this equals 2.98 times To the negative 22nd cm cubed. Our molar mass a potassium bromide Equals 119 zero g Permal. So calculating formula units we have 2.98 Times 10 to the negative 22nd cm cubed, divided by two .75 g per centimeter cubed times one mole, Divided by 119.0 g Times 6.02, 2 times 10 to the 23rd formula units over one mole, And our cm cubed are canceling out. Our g are canceling out. Our moles are canceling out, and that equals 0. formula units. And that is our final answer. Thank you for watching. Bye.