Question

III. Standard Curves When we want to determine the concentration of an unknown solution, we typically measure a certain property of standard solutions with our standard (known) solutions and create what's called a standard curve (or calibration curve). A standard curve relates two variables of certain known solutions. That way, when there is an unknown variable for a certain solution, it can be determined through the standard curve. When plotting standard curves, we always plot the independent variable on the x-axis and the dependent variable on the y-axis. The independent variable is one that is manipulated in the experiment and the dependent variable is the one that is measured and depends on the quantity of the dependent variable. Our goal is to obtain a graph that has a correlation (or R^) value that is as close to 1.000 as possible. Practice 1. Four standard solutions of various concentrations of sodium chloride (NaCI) are prepared. The density of each standard solution is determined by measuring the mass of exactly 15.00 mL of each one (recorded in the table below). The mass of 15.00 mL of an unknown solution is also measured and recorded in the table below. Calculate the density of each standard solution and create a calibration curve. (Hint: since the density is dependent on the concentration of the sodium chloride, Concentration (M) is plotted as the independent variable and Density (g/mL) is plotted as the independent variable.) Use the calibration curve to determine the concentration of sodium chloride of the unknown solution ConcentratioDensity Mass of 15.00 mL Standard of NaCI solution mL 15.455 g 0.500 M 15.869 g 1.000 M 16.326 g 1.500 M 16.768 g 2.000 M 4 Unknowın 16.021 g

Answer 1

Standard	Volume(ml)	Mass(g)	Density= mass/volume (g/ml)	Concentration(M)
1	15	15.455	15.455/15=1.030	0.500
2	15	15.869	15.869/15=1.058	1.000
3	15	16.326	16.326/15=1.088	1.500
4	15	16.768	16.768/15=1.118	2.000
unknown	15	16.021	16.021/15=1.068	unknown

To get the calibration curve we are plotting the data in a graph. As told, density is independent variable so it is plotted along x axis. Concentration is dependent variable so it is plotted along y axis. First four points are plotted and the best fit line is drawn to get the calibration cure. Now to get the unknown concentration, we draw a density constant line from the x axis having density equal to that of unknown solution. Then we get an intersection point on the calibration curve. That point indicates the (density, concentration) coordinate of the unknown solution on the graph paper. So, drawing a peperendicular on the concentration axis from that point gives the corresponding concentration of the solution. Measuring the distance of the point on concentration axis from the orgin, we get the unknown concentration.

The whole thing I' ve uploaded as a graph. The x-axis grid unit is 0.001 and y-axis grid unit is 0.05. So, from the graph you can see, the red triangle on x axis is for unknown solution having density 1.068 g/ml , a line parallel to y axis(density constant line) from that cuts the calibration curve(black) and perpendicular from the intersection is cutting y axis to 1.15 M. So concetration of the unknown solution is 1.15 M.

Concentration(M) 2.5 density(g/ml) 09102 1020 4010 20931 : 11 1.1: 1.11 1.1 至13 114 E