Please write balanced net ionic equations for some of the ionic solids that formed in the precipitation lab. (Use the lowest possible coefficients.

Question

Please write balanced net ionic equations for some of the ionic solids that formed in the precipitation lab. (Use the lowest possible coefficients. Omit states-of-matter from your answer. Enter "NONE" if no reaction occurred.)

a) SrCO_3

b) CaSO_4

c) AlPO_4

Answer 1

a) Sr²⁺ + CO₃²⁻ → SrCO₃
b) Ca²⁺ + SO₄²⁻ → CaSO₄
c) Al³⁺ + PO₄³⁻ → AlPO₄

All three are already at the lowest possible coefficients, and states are omitted as your problem asks.

Why every one of these is 1:1

A net ionic equation keeps only the ions that actually become the solid. Since each of these compounds has a neutral formula built from one cation and one anion, the charges cancel one-for-one and no coefficients are needed:

(+2) + (−2) = 0  |  (+2) + (−2) = 0  |  (+3) + (−3) = 0

Charge has to balance in a net ionic equation just as mass does. Both sides here are zero, so all three check out.

Contrast that with a case where the charges do not match — calcium phosphate needs coefficients to work: 3 Ca²⁺ + 2 PO₄³⁻ → Ca₃(PO₄)₂. The formula of the solid tells you the coefficients directly.

Where the spectator ions went

In lab you mixed two soluble solutions — something like Sr(NO₃)₂ with Na₂CO₃. The Na⁺ and NO₃⁻ stayed dissolved and unchanged on both sides, so they are spectators and drop out. That is the entire difference between a complete ionic equation and a net ionic one, and it is why your answer does not depend on which soluble salts you happened to start from.

Checking these against solubility rules

• Carbonates are insoluble except with group 1 metals and NH₄⁺ → SrCO₃ precipitates.
• Phosphates follow the same rule → AlPO₄ precipitates.
• Sulfates are mostly soluble, with the standard exceptions Ba²⁺, Pb²⁺, Sr²⁺, Ag⁺ — and Ca²⁺, which is the borderline one.

Worth knowing about (b): CaSO₄ is slightly soluble rather than flatly insoluble (K_sp ≈ 4.9 × 10⁻⁵). It does precipitate at the concentrations used in a normal precipitation lab, so the equation above is what you want — but if you saw only faint cloudiness in that tube rather than a solid mass, that is the reason, not an error in your technique. It is the same compound as gypsum and plaster of Paris.

Since none of the three is "NONE," you can be confident all three tubes should have shown a precipitate.