MR PAULLER - Blog

How do you calculate concentration from absorbance?

4/7/2019

Answer:You will use Beer's law. A = εmCl

Explanation: The basic idea here is to use a graph plotting Absorbance vs. Concentration of known solutions. Once you have that you can compare the absorbance value of an unknown sample to figure out its concentration.

http://intro.chem.okstate.edu/ChemSource/Instrument/inst4.htm

You will be applying Beer's law to calculate the concentration.

The equation for Beer's law is: A = εmCl
(A=absorbance, εm = molar extinction coefficient, C = concentration, l=path length of 1 cm)
You should have a data set which was used to create a standard curve. The graph should plot concentration (independent variable) on the x-axis and absorption (dependent variable) on the y axis.

You'll need to add a line of best fit to the data points and determine the equation for the line. The equation should be in y=mx + b form.

y = absorbance (A)
Note: no unit for absorbance
x = concentration (C)
Note: unit is M or mol/L
m = (εm) = slope or the molar extinction coefficient in beers law which has units of M−1cm−1

So A = εmC +b
If you solve for C you should get
C = (A-b)/εm

So if you substract your y-intercept from the absorbance and divide by the slope, you are finding the concentration of your sample.
Here is video of a lab applying this concept.

Waves on a String (HS-PS4-1)

3/18/2019

This simulation from PhET is a great way to review parts of a wave with students. They can explore the relationships of frequency, wavelength and speed of waves. This simulation can be used in place of the classic slinky lab commonly used to teach these concepts.

Exploring Climate Change

3/14/2019

This simulation allows you to explore the relationship of carbon dioxide, water vapor and atmospheric temperatures. Discover how feedback loops connect carbon dioxide, water vapor levels and temperature. The simulation also allows you to learn about albedo and changes in glacial and sea ice. Another cool feature is the ability to manipulate the amount of CO2 being emitted by human activities.

NGSS Connections: HS-ESS2-2 and HS-ESS2-4

Solar Radiation, Carbon Dioxide & the Greenhouse Effect

3/13/2019

Simulation courtesy of: Concord Consortium http://concord.org.

BUILD AN AWESOME BOTTLE BIOSPHERE!

3/6/2019

Make a soda bottle biosphere using materials you have at home and an inexpensive trip to your local aquarium store.

This is a great project to help kids learn about seed germination, photosynthesis, food webs, the water cycle and much more! The project could be even be modified and turned into a great elementary or middle school science fair project!

Completed Bottle Biosphere

Make a soda bottle biosphere using materials you have at home and an inexpensive trip to your local aquarium store.

One example of a famous artificial biosphere is the Biosphere 2 project which was built just outside of Tucson, AZ between 1987-1991. The goal of the project was to make a completely closed artificial ecosystem to demonstrate the possibility of building such systems to support human life in outer space.

Smaller versions like the one pictured to the left usually need some maintenance to keep the plants and animals in them healthy. They are definitely a fun way to help kids learn about seed germination, photosynthesis, food webs, the water cycle and much more! The project could be even be modified and turned into a great elementary or middle school science fair project!

Here is what you will need:
-Two 2-L soda bottles
-Scissors
-Detail Knife / X-Acto (optional)
-Awl
-Plastic packing tape
-Lighter
-Permanent marker
-Cotton balls
-Rubbing alcohol
-Paper towel
-Aquarium rock (thoroughly rinsed)
-Potting soil
-Aquatic plants (Anacharis, duckweed etc.)
-Seeds / mosses etc.
-Small land animals (snails etc.)
-Aquatic animals (fish, snails etc.)

Bottle Biosphere Supplies

Prep Work

Make sure soda bottles are thoroughly rinsed (add small volume water, cap, shake, empty and repeat several times). NEVER use detergents, soaps or other cleaners on any of your supplies!!!
Set out water and leave for 24-48 hours to get rid of chlorine and chloramine
May use chemical treatment to dechlorinate the water
Remove labels from soda bottles (may use rubbing cotton balls/rubbing alcohol to help remove the adhesives

Setting up your ecosystem

Cut the top off of soda bottle #1. Make the cut with an X-Acto knife or scissors. The cut should be made slightly above where the bottle starts to taper in. (See image right)
Cut the bottom off of the bottle #2.
Use the awl and heat it using the lighter. This can be used to punch 8-10 holes into one of the bottle caps.
Screw of the cap with holes onto bottle two.
Stuff paper towel into the neck of bottle two. Use enough paper towel so that when compacted it has a depth of about ¾ inch.
Assemble the bottles.
Add potting soil to the top bottle. Make sure you have the paper towel plug to avoid having sediment drip through the hole punched cap into the lower bottle.
Add enough water to moisten the soil and plant quick growing seeds such as radishes. You can add mosses and other small plants transplanted from your yard.
Add aquarium rock to the lower bottle.
Add water and aquarium plants.
You may want to wait a few days to add snails, fish etc.
Use packing tape to hold the bottles together & keep top in place

Aquarium Bottle with Anacharis, Duckweed, Snails and Fish!

TIPS & Extension Ideas

DO NOT use detergent, soap or other cleaning products
Don’t add water directly from the tap - let it sit out
You can cut an access door and cover with tape
Use two pieces of tape on the top: 12 and 6 o’clock and fold back one side of the tape that is on bottle 2. This way you can peel that tape and open the top like a hinge.
Rotate water on a regular basis to keep water clear - always let water sit out to remove chlorine
Limit number of fish, use very small organisms
You can observe the water cycle, seed germination, talk about food webs, use aquarium test kits to measure pH, ammonia, nitrate levels and more

Osmosis in Red Onion Cells

10/2/2018

This video discusses how osmosis can change the appearance of red onion cells by adding or removing water from the cells.

HS-LS1-1: Awesome Computer Model of Protein Structure & DNA Mutations!

9/25/2018

I recently discovered this model from the Concord Consortium. I love it!

Students can view the processes of transcription and translation for the molecule in its original sequence. The ability to see these steps illustrated so clearly will definitely be helpful to students who can find this topic to be challenging because of its abstractness.

They are also able to edit the DNA molecule. By selecting the Edit DNA button they can type in changes to the DNA sequence. They can also click directly on a nitrogen base (ATGC) and select the type of mutation they want to create in the molecule. They can choose a substitution, deletion or insertion mutation. They can then transcribe and translate their new DNA strand to analyze changes in the RNA and protein (amino acid) sequence.

Another cool feature is that the model shows protein models that illustrate which amino acids are hydrophobic and which ones are hydrophilic.

Calculating the Percent Water in a Hydrate.

9/25/2018

This video demonstrates how to calculate the percent water in copper (II) sulfate pentahydrate by analysis of its chemical formula.

Determining the Activity Series of Metals.

9/20/2018

This video shows an experiment comparing the activity of three metals; Cu, Zn and Mg. The metals are first tested hydrochloric acid, and then hot water to determine their levels of activity.

When is DNA copied? HS-LS1-1

9/18/2018

Answer:
DNA is copied during interphase (S phase) before mitosis and just before meiosis.

Explanation:In human, mitosis produces diploid daughter cells which have the same number of chromosomes as the parent cell.

The four phases of the cell cycle. G1 - the intitial growth phase. S - the phase in which DNA is synthesised. G2 - the second growth phase in preparation for cell division. M - mitosis; where the cell divides to produce two daughter cells that continue the cell cycle.

Meiosis produces gametes (sperm or egg) which are haploid, meaning these cells have half the number of chromosomes as body cells such as skin cells.

DNA is copied using a process called semiconservative replication. The video below shows how DNA is copied and then discusses the processes by which RNA and proteins are made.

I have my students use computer models including "Building DNA" by Gizmos and an animated review of DNA replication. Students are asked to evaluate strengths and weaknesses of the models.

Lessons on this topic can connect with NGSS crosscutting concepts such as "Systems and Sytem Models" by using computer models and "Structure and Function" by focusing on how the shape and structure of DNA allows the molecule to store genetic information.

Hope this helps!