Science Buddies: "Ask an Expert"

Equation # 1

p= m/v for us p= m-292 grams of salt over v- .946 liters of water Basically- 1 cup of salt in 1 quart of water- STOCK

p= 309 or 308.6 = Is this grams per mililiter? Is this the density of the salty water- our stock-?

Next, Equation # 2

new concentration - Volume of stock/(volume of stock + volume of water)

Cups

#1- 3/4 cup stock
#2- 3/4 cup stock & 3/4 cup water- new mix 50% or 1/2 stock
#3- 3/4 cup water & new mix 3/4 cup stock from #2 - 25% or 1/4 stock
#4- 3/4 cup water & new mix 3/4 cup stock from #3 - 12.5% or 1/8 stock
#5- 3/4 water

so, do we divide the 309 by 2, 4 , and 8 for the densities-absolute solutions- for cups 2-4??????????
Is the relative solutions the percentages?????????

Please Help??????? Project due tomorrow!

Hello OrtizR:

It looks like you are well on your way to understanding this experiment.

I would like you to take a look at the units you are using for equation #1. As you have stated the your mass is in grams and your volume is in liters. Therefore, with p=m/v your units would be grams per liter. As p represents the density, this is the density of your stock.

Please let me know if you have any follow-up questions.
Kati

Hello,

I am a parent/teacher helping a student with this experiment.

I read your response to the calculation of density of the stock solution and how to figure out the density of the other concentrations.
It doesn't make sense that the density of the stock which should be 292g of salt in 1183 mL of water (5 cups) will be 292/1183= .246g/mL. If that's the density of the stock solution, it's less than the density of water. The density of the stock should be greater than the density of water because the egg floats in the stock but not the water. Are we supposed to factor in the density of water into this? How do we do that?
How do I explain to the student that the egg will not float in water because it's density is more than the density of water which is 1g/mL but it floats in a solution with less density???
please help!!!
Also, once we find the density of the stock solution, we can find the rest of the cups by taking percentages like 50%, then 25%, then 12.5%?
than you!!

Hi,

Sorry for the confusion about densities. You have to add in the weight of the water when calculating density.

One milliliter of water at 21 C weighs about 1.00 grams. When you add 292 g of NaCl to 1183 ml of water (about 24%) the total volume is increased somewhat and you have a solution with a density between 1.18 and 1.19 g/ml.

I did a search for density of sodium chloride solutions and found a number of sites listing different concentrations of NaCl and the corresponding density. Here's an example: https://www.google.com/url?sa=t&rct=j&q ... m8SHBBIQ0w

Don't be confused by some of the density values because they are given in kilograms per cubic meter. Just divide the value by 1000 to convert it to grams per cubic centimeter, g/ml.

I hope this helps.

Sybee