Hi an old person typed in this molarity chemistry question and i am confused on how they got one of these?

Question

numbers

"3. How many grams of CaCL2 are contained in 225 mL of 0.0350 M CaCl2?

Step 1: Convert to L = 0.225 L

Step 2: Calculate moles = 7.875 x 10^-3 mol CaCl2

Step 3: Molar mass = 111.1 g/mol CaCl2

Step 4: Mass of solute = 0.8749 g CaCl2"

can someone explain to me where they got the 7.875 x 10^3 from?? also the mass of solute?

here is the problem I am trying to do... thats why I was googling and this is the problem that I came to that was just like mine except I didnt understand what they did to get #2 3 and 4

here is my other question that I am trying to do

calculate the molarity of the solution by diliting 100. ml of .375 M of K2Cr2O7 with 875 mL more then water?? can anyone explain this math in a way I can unserstand?

Answer 1

To calculate moles in a given volume of solution, multiply volume by molarity:

(0.225 L) (0.0350 moles/L) = 0.007875 moles of CaCl2.

Notice how the units help guide the calculations. Above, the L units cancel in the multiplication.

Molar mass = 2 * 35.5 + 40.1 = 111.1 g/mol (This is the total of the atomic masses of 2 Cl atoms and 1 Ca atom.)

Mass is calculated from moles by multiplying by molecular weight:

(0.007875 mole) (111.1 g/mol) = 0.875 g CaCl2.

For dilutions, remember V1C1 = V2C2, where V's are solution volumes and C's are concentration. In the second problem, after the 875 mL of water is added, we have 975 mL total solution. Now substitute into the equation:

C2 = (0.375 M) (100 mL) / (975 mL)

C2 = 0.0385 M.

Again, notice the volume units cancel, so it's not necessary in this case to convert volume units to L, even though the concentration units are moles/L.