How do you find ppm of sodium using nutrition label

Im taking a food chem class and I'm not sure if I'm doing this calculation correctly.
I didn't include the whole label, only the parts that I think I need.

Given the following nutrition label, calculate ppm of sodium.

Serving Size 1 cup (100 g)
Sodium 490 mg


This is how I tried doing it.

First I found the % (w/w) of sodium.
490 mg --> .490 g
(.490 g / 100 g) x 100 = 0.49 %
0.49% sodium

Then use the sodium % to find ppm.
x ppm is (x mg / 1000g)

(x mg/ 1000g) x 100 = 0.49%
Solve for x.
x = 4.9 mg

My answer is 4.9 ppm. Does that sound right?

I think that you are off by a factor of 1000.

You are correct that 1 ppm (w/w) = 1 mg/kg. We can use this with the label data directly.

Label: 490 mg Na per 100-g serving.

(490 mg Na/100 g)*(10/10) = 4900 mg Na/1000 g = 4900 mg Na/kg

Therefore, we have

4900 mg Na/kg = 4900 ppm Na

BTW, 1% = 10000 ppm.