Before testing a solution for sulfate ions, a student measured out a portion of the solution using the measuring cylinder shown. (a) Read the volume of solu...
Question 1 Report
Before testing a solution for sulfate ions, a student measured out a portion of the solution using the measuring cylinder shown.
(a) Read the volume of solution in the measuring cylinder, in cm³. [1]
(b) State how the student should place their eye when taking the reading. [1]
(c) The student then added dilute nitric acid followed by aqueous barium nitrate to this portion. State the observation that shows a sulfate ion is present. [1]
(d) Name the piece of apparatus that would give a more accurate measurement of this volume. [1]
Answer Details
This part tests reading a measuring cylinder and the barium test for the sulfate ion. On this cylinder the scale runs from \(0\) at the bottom to \(50\) at the top, and each labelled interval of 10 is split into small divisions, so the reading is taken at the bottom of the meniscus with the eye level with the liquid.
(a) The bottom of the meniscus sits at 27.0 cm3 (accept 26 to 28) [1].
(b) The student should place their eye level with the bottom of the meniscus [1]. Reading from above or below introduces a parallax error, making the value read too high or too low.
(c) Adding dilute nitric acid then aqueous barium nitrate to a portion gives a white precipitate, which shows a sulfate ion is present [1]. The precipitate is insoluble barium sulfate:
\[ \text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s) \]The nitric acid is added first to remove carbonate ions, which would otherwise also give a white precipitate.
(d) A burette or a (graduated / volumetric) pipette would measure this volume more accurately [1], because both have finer graduations than a measuring cylinder.
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