Calculating Initial Concentrations

[PrincessJee]

Homework Statement

I am trying to calculate the initial concentrations in a rate law lab

If I have the following
0.020M Br2 solution, and am using 5.0mL of it
4.0M acetone, and am using 5.0ml of it
1.0M HCl and am using 5.0ml of it
H2O 10.0ml

how do I calculate the initial concentrations of solutions used in each run (this example is the first run only).
After mixing all the reagents the total volume is 25.0mL, and the concentrations need to be calculate according to this volume?

Homework Equations

n=m/M
M=mol/L

The Attempt at a Solution

this is what I am guessing... but not sure if it is correct...
If I have 0.020M Br2
then that means I have 0.020mol of Br2
which means if I use 5 ml of it, my concentration will be (0.020mol/0.005L) = 4.0M

is this correct? I am very confused!

 

[Ygggdrasil]

The equation you have to remember is:
M_iV_i = M_fV_f

For example, if you use have 5.0mL (V_i) of 0.020M Br₂ (M_i) and dilute it to a final volume of 25mL (V_f), then you would setup the equations as follows:

(5.0mL)(0.020M) = (25mL)M_f

You can then solve for the final concentration (M_f) and see that it is 0.0040M.


The approach you tried is wrong because 0.020M means that you have 0.020 mol of Br₂ in 1L. In 5mL, you would have only (0.020 mol/L) * (0.0050 L) = 0.00010 mol. This amount in 25mL gives a concentration of (0.00010 mol)/(0.025 L) = 0.0040 mol/L = 0.0040 M.

You could have immediately figured out that your initial guess (4.0M) was wrong because diluting a solution will never make the concentration increase!

 

[Blueshift5]

I can provide guidance on how to calculate the initial concentrations in your rate law lab. First, you will need to determine the moles of each substance used in the reaction. This can be done by using the formula n=m/M, where n is the number of moles, m is the mass in grams, and M is the molar mass of the substance.

In your first run, you are using 5mL of a 0.020M Br2 solution, which means you have (0.020 mol/L x 0.005 L) = 0.0001 mol of Br2. Similarly, you are using 5mL of a 4.0M acetone solution, which means you have (4.0 mol/L x 0.005 L) = 0.02 mol of acetone. For the 1.0M HCl solution, you have (1.0 mol/L x 0.005 L) = 0.005 mol of HCl.

Next, you will need to calculate the total volume of the mixture after all the reagents are mixed. In this case, it is 25.0mL. Now you can use the formula M=n/V to calculate the initial concentration of each substance. For Br2, it will be (0.0001 mol/0.025 L) = 0.004M. For acetone, it will be (0.02 mol/0.025 L) = 0.8M. And for HCl, it will be (0.005 mol/0.025 L) = 0.2M.

It is important to note that the initial concentrations will be different for each run, as the volumes of the reagents used will vary. Therefore, you will need to repeat these calculations for each run in your experiment. I hope this helps clarify the process for you. If you have any further questions, feel free to ask. Good luck with your lab!