Question

A special flask used in the determination of densities, called a pycnometer, has a mass of 16.3179 g when empty, and it has a mass of 48.0250 g when filled with water at 20.0°C. When this same pycnometer is filled with ethyl alcohol at 20.0°C, it is found to have a mass of 41.3934 g. Find the density of ethyl alcohol at 20.0°C. (Given: at 20.0°C, the density of water is 0.9982 g/mL)

Answer 1

Answer: Density of ethyl alcohol at 20.0°C is 0.7895 g/ml.

Explanation:

Density is defined as the mass contained per unit volume.

$Density=\frac{mass}{Volume}$

Given : Mass of water = mass of pycnometer filled with water - mass of empty pycnometer = (48.0250 - 16.3179)g = 31.7071 g

Volume of pycnometer = volume of water = ?

Density of water = $0.9982g/ml$

Putting in the values we get:

$0.9982g/ml=\frac{31.7071 g}{volume}$

$volume=\frac{31.7071g}{0.9982g/ml}$

$Volume=31.76ml$

Thus volume of  pycnometer = 31.76 ml

Now, Mass of ethyl alcohol = mass of pycnometer filled with ethyl alcohol - mass of empty pycnometer = (41.3934 - 16.3179)g = 25.0755 g

volume of pycnometer = volume of ethyl alcohol =  31.76 ml

$Density=\frac{mass}{Volume}$

Putting in the values we get:

$Density=\frac{25.0755g}{31.76ml}=0.7895g/ml$

Thus density of ethyl alcohol at 20.0°C is 0.7895 g/ml.

Answer 2

The p y c n o m e t e r, filled by water of (approximately) 25.0755 g and

0.9982 g/mL, gives the density of ethyl alcohol as 0.6638 g/mL

How ca the density of a the alcohol be found?

Mass of water that fills the p y c n o m e t e r = 48.0250 g - 16.3179 g = 37.7071 g

$Volume = \mathbf{ \dfrac{Mass}{Density}}$

$Volume \ of \ water= \dfrac{37.7071 \, g}{0.9982 \, g/mL} \approx \mathbf{ 37.775 \, mL}$

Mass of the ethyl alcohol = 41.3934 g - 16.3179 g = 25.0755 g

$Density \ of \ ethyl \ alcohol = \dfrac{ 25.0755 \, g}{37.775\, mL} \approx \mathbf{ 0.6638\, g/mL}$

• The density of the ethyl alcohol is approximately 0.6638 g/mL

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