Density Determinations Class Notes
1. Measurements
Using a graduated cylinder marked off in units of tenths will give you a finer measurement .
When you are recording your measurements, they are made up of digits that you are certain of and one uncertain digit at the far right.
2. Reading the graduated cylinder
When a liquid is placed into a glass container it forms a meniscus a curved surface that is lower in the middle than at the edges. In order to read the graduated cylinder correctly, it must be placed on a stable surface such as the desktop of the work area. Never try to read the graduated cylinder while holding it in your hand. Your eye must be level with the bottom of the meniscus. Do not look down at or up at the bottom of the meniscus.
Image from Meniscus Madness http://www.morrisonlabs.com/meniscus.htm
To determine the volume of liquid use the number that is directly at or below the bottom of the meniscus
Image from ChemPages Laboratory Resources http://jchemed.chem.wisc.edu/JCESoft/Programs/CPL/Sample/modules/gradcyl/grad100mL.htm
100 mL Graduated Cylinder reading 52.8 ml
If the graduated cylinder is marked off in units of tenths, then your volume reading must go to the hundredth place.
Images from ChemPages Laboratory Resources http://jchemed.chem.wisc.edu/JCESoft/Programs/CPL/Sample/modules/gradcyl/grad10mL.htm
10
mL Graduated Cylinder reading 6.65 mL
Density
Density is a physical property of a substance defined as mass per unit volume and determined by dividing the mass of the substance by its volume.
Density = mass/volume
( D = m/v)
Units for density of a solid substance is grams/cm3.
Units of density for liquids are grams/mL or grams/cc or grams/cm3.
Density of a Liquid
The density of a liquid is determined by using a clean graduated cylinder to hold the liquid. The mass of the liquid is calculated by using weighing by difference (ie [Mass of container + liquid] - mass of container = mass of liquid). The volume of the liquid is determined by reading the graduated cylinder at the bottom of the meniscus.
The graduated cylinder must be clean and dry before taking these measurements. If the graduated cylinder is dirty inside, you are getting a volume measurement that may not be correct, since the dirt also has a volume that may be included in the volume measurement. If the cylinder is wet inside, you will get a volume larger than the true volume due to the water being present in the bottom of the graduated cylinder. Always make sure the outside of the graduated cylinder is dry before placing it on the balance.
Density of a Regular Solid
A regular solid is something that we can measure easily with the centimeter ruler. The mass of the regular solid is easily obtained by placing it on the balance. The volume of the CUBE is determined by measuring the height x width x length. (h x w x l)
Density of an
Irregular Solid
You cannot simply measure an irregular solid and
calculate its volume without using calculus. An example of an irregular solid
is a screw. Without knowing how to determine the area where the threads are,
you cannot simply measure its length and diameter. We will be using a technique
called liquid displacement. By determining the amount of liquid
displaced by a solid, you can determine its volume. The irregular shape must be
completely covered by the liquid in order to get the correct volume of the
solid. Mass of the irregular shape is determined by weighing by difference.
(mass of container + water
+ irregular solid) - (mass of container + water) = mass of irregular solid
The volume is determined by reading the volume of a
specified amount of water, then placing the irregular solid in the water and
reading the volume again. This is called liquid displacement.
(Volume of liquid + irregular solid) - volume of liquid = volume of irregular solid