A pycnometer is a device used to measure density. It weighs 20.455 grams when empty and 31.486 grams when filled with water. Pieces of an alloy are placed into the empty, dry pycnometer. The mass of the alloy and the pycnometer is 28.695 grams. Water is added to the alloy to exactly fill the pycnometer. The mass of the pycnometer, water, and alloy is 38.689 grams. Assume the density of the water is 1.000 g/cm3, calculate the density (in g/cm 3 and in g/mm3) of the alloy

Explanation:

We know that mass of empty pycnometer  , mass of the pycnometer filled with water is  and density of water is  

Firstly we have to find the mass of water, using the mass of empty pycnometer and mass of pycnometer filled with water.

Chapter 1, Problem 64QP is solved.

View this answer

View a sample solution

1. for the first question, we must find the mass of the anhydrous salt, mgso₄. the molar mass for mgso₄·7h₂o is 246.47 g/mol, while that of mgso₄ is 120.37 g/mol.7.834 g mgso₄·7h₂o * 1 mol mgso₄·7h₂o/246.47 g * 1 mol mgso₄/1 mol mgso₄·7h₂o * 20.37 g mgso₄/mol = 0.647 g mgso₄

2. mass of water = mass of sample - mass of anhydrous salemass of water = 7.834 - 0.647 = 7.19 gpercent water in hydrate = 7.19/7.834 * 100 = 91.74%

"standard molar enthalpy:

h2o ( liquid water ) : - 285.8 kj/mole

h2o ( water vapor ) : - 241.8 kj / mole.

δhrxn = - 241. 8 - ( - 285.8 )= -241.8 + 285.8 = 44.0

answer: 44.0 kj"