A laboratory records the concentration of a chemical in a solution every half-hour. The readings form an arithmetic progression. The concentration at the first reading is 5.0 g L−1 and at the 9th reading it is 2.8 g L−1.
(i) Find the common difference of the progression.
(ii) Obtain an expression for the concentration Cn at the nth reading.
(iii) After how many readings will the concentration reach exactly 1.0 g L−1, if at all?