Gypsum is a mineral that is used in the construction industry to make drywall (sheetrock). The chemical formula for this hydrated compound is CaSO4 2 H20. A hydrated compound contains water molecules within its crystalline structure. Gypsum contains 2 moles of water for each 1 mole of calcium sulfate. a. What is the gram formula mass of CaSO4. 2H2O? b. Show a correct numerical setup for calculating the percent composition by mass of water in this compound. Record your answer. c. The empirical formula for naphthalene is C5H4 and the molecular mass of naphthalene is128 grams/mole. What is the molecular formula for naphthalene? d. What is the total number of moles in 80.0 grams of C2H5Cl (gram-formula mass = 64.5 grams/mole) ? c. What is the mass of 4.76 moles of Na3PO4 (gram - formula mass = 164 grams/mole) ?

a. The gram formula mass of CaSO4.2H2O equals 172 grams/mole

b. The percentage by mass of H2O in CaSO4.2H2O is 20.9%

c. The molecular formula for naphthalene is C10H8

d. The total number of moles in 80.0grams of C2H5Cl is 1.24 moles

e. 4.76moles of Na3PO4 contains 780.64 grams of Na3PO4

Explanation:

RMM = Relative Molecular Mass

a. To calculate the gram formula mass of CaSO4.2H2O, you would have to add the atomic mass of the individual elements that make up to compound. Here are the atomic masses of individual elements present in gypsum; Ca=40, S=32, O=16, H=1.

CaSO4.2H2O = 40+32 + (16x4) + 2 ((1x2) + 16) = 172 grams/mole

b. % by mass of H2O in gypsum = (RMM of 2H2O/RMM of gypsum) x 100

RMM of 2H2O = 2 ((1x2) + 16) = 36 gram/mole

RMM of CaSO4.2H2O (gypsum) = 172 grams/mole

% by mass of H2O in gypsum = (36/172) x 100 = 20.9% of H2O

c. The empirical formula of a compound only shows the simplest ratio of atoms in a particular compound not the actual number of atoms. The actual number of atoms in a compound is shown in its molecular formula, which can be used to calculate the RMM of the compound. A specific number (n) multiplied by the number of atoms in an empirical formula gives the molecular formula. (C=12, H=1)

(Empirical formula) n = Molecular formula

(Total sum of atomic masses of elements in empirical formula) n = Total sum of atomic masses of elements in molecular formula

RMM of naphthalene = 128 grams/mole

((12x5) + (1x4)) n = 64n

64n = 128

n = 128/64 = 2

Therefore, (C5H4) 2 = C (5x2) H (4x2) = C10H8

d. Number of moles of C2H5Cl = mass of C2H5Cl/RMM of C2H5Cl

Say number of moles of C2H5Cl = y

Mass of C2H5Cl = 80.0 grams

RMM or gram-formula mass of C2H5Cl = 64.5 grams/mole

y = 80.0/64.5 = 1.24 moles

e. Number of moles of Na3PO4 = mass of Na3PO4/RMM of Na3PO4

Number of moles of Na3PO4 = 4.76 moles

gram-formula mass of Na3PO4 = 164 grams/mole

mass of Na3PO4 = x

4.76 = x/164

x = 4.76 x 164 = 780.64 grams