You are given a sample of limestone, which is mostly CaCO3, to determine the mass percentage of Ca in the rock. You dissolve the limestone in hydrochloric acid, which gives a solution of calcium chloride. Then you precipitate the calcium ion in solution by adding sodium oxalate, Na2C2O4. The precipitate is calcium oxalate, CaC2O4. You find that a sample of limestone weighing 128.3 mg gives 140.2 mg of CaC2O4. What is the mass percentage of calcium in the limestone

34.15% is the mass percentage of calcium in the limestone.

Explanation:

Mass of precipitate that is calcium oxalate = 140.2 mg = 0.1402 g

1 mg = 0.001 g

Moles of calcium oxalate =

1 mole of calcium oxalate have 1 mole of calcium atom.

Then 0.001095 moles of calcium oxalate will have 0.001095 moles of calcium atom.

Mass of 0.001095 moles of calcium :

0.001095 mol × 40 g/mol = 0.04381 g

Mass of sample of limestone = 128.3 mg = 0.1283 g

Percentage of calcium in limestone:

34.15% is the mass percentage of calcium in the limestone.

The answer is C, 7.5.

This is because you know that these are the same objects, since it’s asking to find the scale factor. You are able to correlate AB to EF, and that shows a scale UP of 1.5, from 2 to 3. With this scale factor of 1.5, we know that EH will be 1.5 times bigger than AD. So 1.5 x 5 is equal to 7.5.