Which of the following best explains why most chemical reactions proceed more quickly when the concentrations of reactants are increased?
a. the increased concentration increases the number of collisions between molecules.
b. the products of chemical reactions are more stable at higher concentrations.
c. at higher concentrations, reactant molecules move more quickly.
d. at higher concentrations, product molecules are able to catalyze the reaction.

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.