18 (A410U10-1) Examiner only 15. The wide range of mobile technology available to us today has relied on the development of ways to store and utilise energy efficiently. Many mobile devices use batteries based on lithium ions as they have a large charge density. (a) A lithium ion battery can sustain a current over an extended period of time so that a total charge of 1.2 × 104 C has flowed. Calculate the mass of lithium that has reacted in this time, giving your answer to an appropriate number of significant figures. [3] Mass = ...................................................... g (b) Many lithium ion batteries contain electrodes made of compounds of formula LiaMb(PO4)c where M can be one of a range of metals from the first row of the d-block elements. A university student undertook a series of experiments to analyse the compound used as an electrode in a given lithium ion battery. The relative formula mass of the compound was approximately 158. 2.70 g of the solid were dissolved in hot acid and a solution containing calcium ions was added. All the phosphate ions were precipitated from the hot mixture as calcium phosphate (Mr 310.3). The calcium phosphate was washed and heated to constant mass, with 2.66 g produced. A small amount of the original compound was dissolved in hot solvent to give a solution of concentration 1.90 × 10−6 mol dm−3. Atomic absorption spectroscopy was used to measure the concentration of lithium ions in this solution and it was found to be 13.2 µg dm−3. (i) Explain why the solid calcium phosphate precipitate must be heated to constant mass. [1] (ii) Find the value of c, the number of phosphate ions present in the molecular formula. You must show your working. [3] c = ...................................................... © WJEC CBAC Ltd. (A410U10-1) Turn over. 19 Examiner only (iii) Find the value of a, the number of lithium ions present in the molecular formula. [2] a = ...................................................... (iv) Identify the first row d-block element M, and the number of atoms of this present in the molecular formula, b, and hence write the molecular formula of the compound. [2] Molecular formula ............................................................................................... © WJEC CBAC Ltd. 11