Can someone please answer this question

Question

Magnesium carbonate and phosphoric acid react to form products in a double displacement reaction.

Write the balanced chemical equation for this reaction. Make sure to correctly format the formulas with subscripts if necessary, but do not include phases.

   ________  __________

Answer 1

Hi Janet!

To write chemical formulas of ionic compounds from the name:

1. Start with the symbol of the metal
2. Mg
3. Determine if the metal is a main group metal or a transition metal.
4. Main group metals are groups 1 and 2.
5. Mg is a main group metal in group 2.
6. Determine the charge of the metal and write it down by the symbol.
7. Main group metals
8. Group 1 metals will have a +1 charge.
9. Group 2 metals will have a +2 charge.
10. Transition metals will have varying charges. **The charge must be given to you in parentheses.**
11. Mg is in group 2, so its charge is +2.
12. Look at the second half of the name.
13. If the name ends in -ide, you have one nonmetal that is an anion.
14. If the name ends in -ate or -ite, you have a polyatomic ion. **You must have these memorized or the teacher will have to give them to you.
15. The name of the first compound ends in -ate, so that tells me that we will have a polyatomic ion.
16. Write the symbol and charge of the anion.
17. If the name ended in -ide and you have one non-metal, then you can use the periodic table to determine the charge of the anion.
18. Group 15/5A elements will have a -3 charge.
19. Group 16/6A elements will have a -2 charge.
20. Group 17/7A elements will have a -1 charge.
21. If you have a polyatomic ion, then the charge and formula will be memorized (or given to you by your teacher).
22. Carbonate is CO₃ ^-2
23. Add the charges together to see if they cancel out/add to 0.
24. If they do, no additional subscripts are needed.
25. Mg ⁺² CO₃ ^-2
26. When these two charges are added together, they cancel out. There are no additional subscripts to add. The final formula is MgCO₃
27. If they do not, do the criss-cross method.
28. You do NOT bring down the +/- signs, you only bring down the numbers.
29. You never write "1" as a subscript in chemistry.
30. EX1: Sodium sulfate
31. Na⁺ SO₄ ^-2
32. The +1 by the Na will technically be brought down by the SO₄, but we do not write "1" as a subscript in chemistry, so it disappears.
33. The 2 by the SO₄ will be brought down by the Na.
34. Na₂SO₄
35. EX2: Iron (III) nitrate
36. Fe ⁺³ NO₃ ^-1
37. The 3 by the Fe will be brought down by the NO₃. Since you are bringing the 3 by a polyatomic ion, you should put the polyatomic ion in the parentheses. You do *not* change the 3 by the NO_3.
38. The 1 by the NO₃ will technically be brought down by the Fe, but we do not write it in the final answer.
39. Fe(NO₃)₃

Now it's time to write the formula of the acid.

1. Most acids begin with H, so write the symbol and charge of H, which is H+.
2. Does the acid have a prefix of hydro-?
3. No ---> you probably have an oxyacid which will have a polyatomic ion.
4. Phosphoric acid does NOT have a prefix of hydro, so we know it will have a polyatomic ion.
5. Yes --> you probably have just one non-metal anion.
6. If there was no prefix of hydro, look at the ending of the acid name.
7. Ends in "ic acid" --> polyatomic ion ends in "ate"
8. Ends in "ous acid" --> polyatomic ion ends in "ite"
9. **Here's a helpful saying: "I had a bITE and it was deliciOUS, but I ATE more and got sICk"
10. The name of our acid here is phosphoric acid, so because it ends in "ic acid", this means that our polyatomic ion ends in "ate" so it'd be phosphate.
11. Write the symbol and charge of the polyatomic ion or (if there WAS a prefix of hydro, then write the symbol) and charge of the non-metal.
12. Phosphate is PO₄ ^-3
13. Add the charges together to see if they cancel out/add to 0.
14. If they do, no additional subscripts are needed.
15. If they do not, do the criss-cross method.
16. H+ PO₄ ^-3
17. H₃PO₄

We now have the formulas for the two reactants. MgCO₃ + H₃PO₄

The question explains that these reactants undergo a double-displacement/replacement reaction. This means that the two cations (Mg and H) will switch places. The Mg will now be combined with PO4 and the H will now be with the CO3. We will now have to write *brand new formulas* for these two products. Students often think that they can keep the original subscripts for the reactants, but this is not true! You must start over and write brand new formulas.

1. Mg⁺² PO₄ ^-3 ---> Mg₃(PO₄)₂
2. H⁺ CO₃ ^-2 ----> H₂CO₃
3. MgCO₃ + H₃PO₄ --> Mg₃(PO₄)₂ + H₂CO₃
4. Now it's time to balance
5. Balance all elements that are NOT O, H.
6. Balance H.
7. Balance O

FINAL ANSWER: 3MgCO₃ + 2H₃PO₄ --> Mg₃(PO₄)₂ + 3H₂CO₃