Homework Help: Chemistry, equation and formula

1. Dec 24, 2008

Dell

dont know why all the index numbers came above the letters in my post but they are meant to be below.

250ml of Mg(NO$$_{3}$$)$$_{2}$$ with a density of 45g/L are mixed with 75 ml Na$$_{3}$$PO$$_{4}$$ with a density of 30g/l.
what is the mass of the product Mg$$_{3}$$(PO$$_{4}$$)$$_{2}$$

what i did was, using the densities, found out the masses of the molecules and then using the masses found out the amount of given mol's of each.
what i get is

0.071 mol Mg(NO$$_{3}$$)$$_{2}$$
0.0137 mol Na$$_{3}$$PO$$_{4}$$

giving me a ratio of about 5:1

so

5Mg(NO$$_{3}$$)$$_{2}$$+ Na$$_{3}$$PO$$_{4}$$ ----> Mg$$_{3}$$(PO$$_{4}$$)$$_{2}$$

problem comes here, i need to balance the equation to continue,
but i have missing elements?? what do i do?

once i have balanced the equation i can see how many mols of Mg$$_{3}$$(PO$$_{4}$$)$$_{2}$$ i have and then i fan work out its mass.

how do i add up the missing elements?

2. Dec 24, 2008

Staff: Mentor

Don't use tex for single character formatting, use [ sub][ /sub] and [ sup][ /sup] tags (no spaces after [).

Something is wrong, these solutions will have a density of around 1000 g/L, numbers you have posted don't make sense. Are you sure you have copied numbers/question correctly?

This is double exchange reaction with two reactants and two products. Think in terms of ions present in the solution before reaction. Precipitate removes some of these ions, others are left. Those left define other product.

3. Dec 24, 2008

Dell

so my balanced equation will come to ----> Mg3(PO4)2 + something,, is this right,,
how can i know what the "something" is? can i just add in the missings ones to balance it? how can i know how to write them?

4. Dec 24, 2008

Staff: Mentor

How did you know your reactants formulas were Mg(NO3)2 and Na3PO4?

5. Dec 24, 2008

Dell

that was given in the question

6. Dec 24, 2008

Staff: Mentor

Do you know any systematic method for determining compound formula? Especially for salts the method is relatively simple and is based on the fact that salt molecule must be neutral.

7. Dec 24, 2008

Dell

havent learnt that yet, is there no other way i can solve this??? are my assumptions thus far correct??
could you tell me what the formula is meant to be so i can continue with the excercise

8. Dec 24, 2008

Staff: Mentor

Even if I will give you the formula of the compound, your data so far (numbers given as densities) don't make sense.

Are you sure these are densities, and not amounts of substance dissolved per litre of the solution?

Can you name the substances mixed?