If .800 g BaSO4 was recovered from 25.0 mL of BaCl2, how many Ba2+ remained unreacted?

Question

Assume 100 % of the BaSO4 was recovered

the concentration of BaCl2 is .25 M

Answer 1

You can't tell from that. You didn't say how concentrated the BaCl2 solution was. To solve this problem, first multiply the molarity (moles per liter) of the solution by the volume of the soution in liters (0.025 in this case). That gives you the number of moles of BaCl2 you started with.

Then, calculate the molar mass of BaSO4. You can find the atomic weights of the elements on your periodic table. The molar mass is the sum of the atominc masses--the mass of Ba, the mass of S, and four times the mass of O. Add all that up and you've got the molar mass. This is what one mole of BaSO4 weighs, or the grams per mole.

Take the reciprocal of the molar mass (1/molar mass) to get the number of moles per gram, and multiply this by 0.8 grams to get the number of moles recovered. Then to get an absolute value for the amount of Ba2+ unreacted, just subtract the moles recovered from the starting moles.

To get the percent unreacted, take the moles unreacted from the last step, divide by the starting moles, and multiply by 100%.