25 points + ! how do you calculate the number of molecules in a substance? for example, al2o3. (the numbers are subscripts) explain simply, to a point where a grade 9 can understand.

What layer of the atmosphere is directly above the troposphere?

What mass of natural gas (ch4) must you burn to emit 276 kj of heat?

Tooth enamel consists mainly of the mineral calcium hydroxyapatite, ca_10(po_4)_6(oh)_2. trace elements in teeth of archaeological specimens provide anthropologist with clues about diet and diseases of ancient people. students at hamline university measured strontium in enamel from extracted wisdom teeth by atomic absorption spectroscopy. solutions with a constant total volume of 10.0 ml contained 0.726 mg of dissolved tooth enamel plus variable concentrations of added sr. added sr find the concentration of sr in the 10 ml sample solution in parts per billion = ng/ml. find the concentration of sr in tooth enamel in parts per million = mu g/g.