a little trouble with these, kindly help please

Question

1) How many grams of vitamin B12 are present in a 100.0 mL blood serum sample with a vitamin B12 concentration of 148 ng/L?
A)1.48 x 10-8 g
B)1.48 x 10-5 g
C)1.48 g
D)1.48 x 10-9 g

2)Sodium fluoride is added to drinking water. A 25-L sample of city water contains 0.018 g NaF. Express this concentration in molarity.
A)0.018 M
B)18 M
C)0.00072 M
D)0.000017 M

3)If 48 L of city water contains 0.030 g sodium fluoride, what is the concentration in parts per thousand?
A)1.4 ppt
B)0.00063 ppt
C)16 ppt
D)0.0025 ppt

4)15.0 g of KCl is dissolved into a solution with a total volume of 250.0 mL. What is the molarity of the solution?
A)0.804 M
B)1.24 M
C)0.200 M
D)3.75 M

5)An egg yolk with the volume of 3.75 mL contains 5.00 µg vitamin D (cholecalcifoerol). What is the concentration of vitamin D in mg/dL? (1 µg = 10-6 g and 1 dL = 10-1 L)
A)0.0188mg/dL
B)0.750 mg/dL
C)0.133 mg/dL
D)1.30 mg/dL

Answer 1

1.
148 ng/L = 148E-9 g/L.
Then g/L is 1.48E-9 g/L x #L = ?

2.
mols NaF = grams/molar mass
M = mols/L

3.
You have 0.030 g in 48,000 g and you want to convert that to g in 1000. So
0.030 g x 1000/48000 = ?

4.
mols = grams/molar mass
Then M = mols/L

5.
5 ug x (1 mg/1000 ug) = ? mg
3.75 mL x (1 L/1000 mL) x (10 dL/L) =? dL
Then mg/dL = ?

Answer 2

on 3. my calculator gives me a crzy answer, 6.25e-4 this cant be right, can it

Answer 3

for others facing these problems,
1.A
2.D
3.B
4.A
5.C