neon has 3 different isotopes, neon-20, neon-21, and neon-22. the first isotope has a mass 19.992 amu and an abundance of 90.60% . the second isotope (neon-21) has an abundance of 0.21%. the third isotope has a mass 21.991 amu and an abundance of 9.20%. what is the mass of the second isotope?

The organism shown is a free-living one that is anchored to the bottom of ponds and streams during one stage of its life cycle what is the common name for the group to which this organism belong

Asample of tin (ii) chloride has a mass of 0.49 g. after heating, it has a mass of 0.41 g.  what is the percent by mass of water in the hydrate?   %

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.