All-purpose fertilizers contain the essential elements nitrogen, phosphorus, and potassium. A typical fertilizer carries numbers on its label, such as "5-10-5". These numbers represent the % N, % P₂O₅ and % K₂O, respectively. The N is contained in the form of a nitrogen compound, such as (NH₄)₂SO₄, NH₄NO₃, or CO(NH₂)₂ (urea). The P is generally present as a phosphate, and the K as KCl. The expressions % P₂O₅ and % K₂O were devised in the nineteenth century, before the nature of chemical compounds was fully understood. To convert from % P₂O₅ to % P and from % K₂O to % K, the factors 2 mol P/mol P₂O₅ and 2 mol K/mol K₂O must be used, together with molar masses.

(a) Assuming three-significant-figure precision, what is the percent composition of the "5-10-5" fertilizer in % N, % P, and % K?

(b) What is the % P₂O₅ in the following compounds (both common fertilizers)? (i) Ca(H₂PO₄)₂; (ii) (NH₄)₂HPO₄.

(c) In a similar manner to the "5-10-5" fertilizer described in this exercise, how would you describe a fertilizer in which the mass ratio of (NH₄)₂HPO₄ to KCl is 5.00:1.00?

(d) Can a "5-10-5" fertilizer be prepared in which (NH₄)₂HPO₄ and KCl are the sole fertilizer components, with or without inert nonfertilizer additives? If so, what should be the proportions of the constituents of the fertilizer mixture? If this "5-10-5" fertilizer cannot be prepared, why not?