The mean serum urate concentration (from uric acid) in a sample of urine was measured to be 64 mg/L. If a 2.0 mL aliquot of the urine sample is analyzed, what is the total mass of the serum urate present in the aliquot?

Question

The mean serum urate concentration (from uric acid) in a sample of urine was measured to be 64 mg/L.  If a 2.0 mL aliquot of the urine sample is analyzed, what is the total mass of the serum urate present in the aliquot?

Quizplus · Accepted Answer

11ecba2d_11f0_c87d_a3bf_b756bfed040a_MD0008_00 Solution concentrations are sometimes expressed in terms of the mass of a given substance per unit volume of solution. When such a concentration is known, if an aliquot of known volume is taken from the solution, the mass of the substance contained in the aliquot can be calculated by multiplying the aliquot volume by the solution concentration provided that both values are expressed with respect to the same units of volume. The concentration of urate in the urine sample was measured to be 64 mg/L, and the aliquot taken was 2.0 mL. To determine the mass of urate in the aliquot, one measurement must be converted to the same units of volume as the other. Converting the aliquot volume from mL to L gives: 11ecba2d_11f0_c87e_a3bf_0383f54f44eb_MD0008_00 Multiplying the converted aliquot volume by the given urate concentration yields the mass of urate contained in the aliquot (in milligrams): 11ecba2d_11f0_ef8f_a3bf_875e7f887de2_MD0008_00 Stating the mass of the urate in grams is accomplished by an additional metric conversion: 11ecba2d_11f0_ef90_a3bf_596c1e0a4cb4_MD0008_00 Rounding the answer to two significant figures and expressing it in scientific notation gives 1.3 × 10⁻⁴ g. (Choice A) This value is obtained if the urate concentration is divided, instead of multiplied, by the aliquot volume. (Choice C) This value results from doing both of two errors: not performing a unit conversion on either the aliquot volume or the urate mass, and dividing the urate concentration by the aliquot volume instead of multiplying. (Choice D) This value is off by a factor of 1000 and is likely the result of not performing a unit conversion on either the aliquot volume or the urate mass. Educational objective: Solution concentrations can be expressed as the mass of a given substance per unit of solution volume. The mass of a substance contained in an aliquot can be calculated by multiplying the aliquot volume by the solution concentration when both values are expressed with respect to the same units of volume.

The Mean Serum Urate Concentration (From Uric Acid) in a Sample