Crystal violet standard solutions were prepared and their absorbance at a certain wavelength was measured using a spectrophotome
ter. The path length was 1 cm. The data was plotted using Excel and a line was fit to the points (see below). A sample of unknown concentration was found to have an absorbance of 0.652 at the same wavelength. (a) What is the molar absorptivity of this compound at the certain wavelength? (b) What is the concentration of the unknown sample solution?
1 answer:
Answer:
(a) 128459 L/mol·cm
(b) 5.11 x 10⁻⁶ M
Explanation:
This question was incomplete, it is missing the Excel plot and the line fit, please see attached image of the question.
Crystal violet standard solutions were prepared and their absorbance at a certain wavelength was measured using a spectrophotometer. The path length was 1 cm. The data was plotted using Excel and a line was fit to the points (see below). A sample of unknown concentration was found to have an absorbance of 0.652 at the same wavelength. (a) What is the molar absorptivity of this compound at the certain wavelength? (b) What is the concentration of the unknown sample solution?
(a) This problem we calls for the use of Beer´s law:
A = εcl
where A is the absorbance, ε is the molar absorptivity, c the concentration, and l the path of the cuvette used.
This equation when plotted A vs c will give a straight line since A is a function of c
Thus we can determine ε by using the calibration curve in the Excel plot
y = 128459 x - 0.0042
the slope of this equation is the molar absorptivity since the length is one, so :
ε = 128459 L/mol·cm
For part (b)
A = 0.652 = 128459 L/mol·cm x c x 1 cm - 0.0042
⇒ 0.652 +0.0042 = 128459 c = 5.11 x 10⁻⁶ M
In absorbance spectrophotometry it is necessary to work with very dilute solutions to avoid deviations characteristic of high concentrations. One can always dilute the unknown and knowing exactly the dilution factor one can trace back the value for the a more concentrated solution.
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Answer:
The attractive force is negative and MgO has a higher melting point
Explanation:
From Couloumb's law:
Energy of interaction, E = k
where q1 and q2 are the charges of the ions, k is Coulomb's constant and r is the distance between both ions, i.e the atomic radii of the ions.
If you look at Coulomb's law, you note that in the force is negative (because q1 is negative while q2 is positive).
In addition to that, the compounds MgO and NaF have similar combined ionic radii, then we can determine the melting point trend from the amount of energy gotten
The melting point of ionic compounds is determined by 1. charge on the ions 2. size of ions. while NaF has smaller charges (+1 and -1), MgO (+2 and -2) has larger charges and greater combined atomic radii. This implies that the compound with greater force would have a higher melting point.
Hence the compound MgO would have a higher melting point than NaF.