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Which reaction will result in a precipitate?

(A) FeCl2(aq) + Na2S(aq)--->

(B) H2SO4(aq) + NaOH(aq)--->

(C) KCl(aq) + CaBr2(aq)--->

(D) Zn(NO3)2(aq) + Hl(aq)--->

Solution: These are double replacement reactions. To predict the products, we can use the formula AB+CD---> AD+ CB

Let's first predict the products for each reaction

(A) FeCl2(aq) + Na2S(aq)---> FeS + NaCl

(B) H2SO4(aq) + NaOH(aq)---> HOH + Na2SO4

(C) KCl(aq) + CaBr2(aq)---> KBr + CaCl2

(D) Zn(NO3)2(aq) + Hl(aq)---> ZnI2 + HNO3

A precipitate is a solid product that results from the mixture of the two aqueous solutions. To predict the precipitate, we need to memorize solubility rules. A substance that is INSOLUBLE, will be the precipitate.

Let's look at some solubility rules:

  • If a substance has Group 1 ions ( Example: Na+, Li+, K+), it is always soluble.

  • If a substance has ammonium, NH4+, it is always soluble.

  • If a substance contains carbonate (CO3^2-), phosphate (PO4^3-) or sulfide (S^2-), it is insoluble unless its bonded to ammonium or Group 1 ion.

FeS is insoluble because it contains sulfide bonded to iron. Therefore, reaction A would give a precipitate.

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