The following data were obtained when a Ca2+ ion selective electrode was immersed in a series of standard solutions.
Ca2+ (M)
3.38E-5
3.38E-3
3.38E-1
E(mV)
-74.8
-18.7
37.7
Calculate the concentration of a sample that gave a reading of -22.5 mV
I suggest you plot the data, then read concn from the graph at -22.5 mV.
Did that multiple times and the answer is still incorrect. The equation I computed was y=252.346x+(-47.318)
I've also tried plugging in -22.5 as y and solved for x and still got .098M which was incorrect
Just a glimmer of hope here but it might be worth trying. You data has three significant figures and your answer only two; these on-line data bases are notorious about s.f. Try adding another place to your answer.
the correct answer is 2.45E-3M but I have no clue how that's possible
To calculate the concentration of a sample that gave a reading of -22.5 mV, we can use the Nernst equation. The Nernst equation relates the electrode potential (E) to the concentration of the analyte ions (Ca2+ in this case). The equation is as follows:
E = E° - (0.0592/n) * log(Ca2+)
where E is the measured electrode potential, E° is the standard electrode potential, n is the number of electrons involved in the reaction (in this case, 2 since Ca2+ has a 2+ charge), and log refers to the base 10 logarithm.
In order to use the Nernst equation, we need to determine the standard electrode potential (E°). We can do this by looking at the data provided and finding the point where the concentration (Ca2+) is equal to 1. This is typically the standard concentration used for electrode potential measurements.
In the given data, the concentration of 3.38E-3 M corresponds to an electrode potential of -18.7 mV. We can use this data point to determine the standard electrode potential (E°).
E = E° - (0.0592/2) * log(Ca2+)
-18.7 = E° - (0.0296) * log(3.38E-3)
Simplifying the equation:
E° = -18.7 + (0.0296) * log(3.38E-3)
Now that we have the standard electrode potential (E°), we can use it to calculate the concentration of the sample that gave a reading of -22.5 mV.
-22.5 = E° - (0.0296) * log(Ca2+)
Now, we can rearrange the equation to solve for the concentration (Ca2+):
Ca2+ = 10^((E° - 22.5)/0.0296)
Substituting the value of E° we calculated earlier:
Ca2+ = 10^((-18.7 + 0.0296 * log(3.38E-3) - 22.5)/0.0296)
Using a scientific calculator, we can evaluate this equation to find the concentration of the sample that gave a reading of -22.5 mV.
Note: It is important to use the proper unit for E° in the calculations. If E° was given in volts (V), then it needs to be converted to millivolts (mV) by multiplying by 1000 in the equation.