The concentration of the appetite regulating hormone ghrelin is about 1.3 x 10-9 M in the blood of a fasting person. How many molecules of ghrelin are in 1 L of blood?

7.826*10^14 molecules of ghrelin

Explanation:

Molarity of ghrelin = number of moles of ghrelin/volume of blood

number of moles of ghrelin = Molarity of ghrelin * volume of blood = 1.3*10^-9 * 1 = 1.3*10^-9 moles

From Avogadro's law

1 mole of a substance (ghrelin) contains 6.02*10^23 molecules

1.3*10^-9 moles of ghrelin contains 1.3*10^-9 * 6.02*10^23 = 7.826*10^14 molecules of ghrelin