Answer:
3.6x10^21 Ca ions
Step-by-step explanation:
The definition of molar (M) is moles/liter. A 1.50M Calcium chloride (CaCl2) solution has 1.5 moles of CaCl2 in every liter. The number of moles in 4 ml can be calculated by including a conversion factor to account for the fact that concentration is expressed in liters, but the sample size is in ml. If we converted the 1.5 moles/liter into moles/ml, we could just multiply the two to find the number of moles in the 4 ml aliquot.
Bt definition, 1000ml = 1 liter
We can arrange this into either of two conversion factors:
1) (1000ml/1 liter), or
2) (1 liter)/(1000 ml)
I'll use the second option, because the terms will cancle using multiplication. (1000ml/iliter) can be used, but it would require a division, which I dislike, given a chance to multiply.
(1.5 moles CaCl2/1 liter)*(4 ml)*(1 liter/1000 ml) = 0.006 moles CaCl2 in 4 ml of 1.5M CaCl2 solution.
1 mole is 6.02x10^23 molecules of CaCl2. Multiply this conversion facor times the moles CaCl2 to find molecules of CaCl2:
(0.006 moles CaCl2)(6.02x10^23 molecules/mole CaCl2) = 3.6x10^21 molecules CaCl2.
Since there is 1 Ca per molecule, we'll have 3.6x10^21 Ca ions.