To make a 1000 mg/L magnesium (Mg) standard solution in 100 mL of 5% nitric acid, you can use magnesium nitrate (Mg(NO3)2). The formula weight of magnesium nitrate is 148.31 g/mol.

Here's the calculation for making 1000 mg/L Mg standard solution:

1000 mg/L = 1000 mg / 100 mL = 10 mg/mL

To make 10 mg/mL solution, you need 10 mg / 148.31 g/mol = 0.06767 g of Mg(NO3)2.

So, you can weigh 0.06767 g of magnesium nitrate and dissolve it in 100 mL of 5% nitric acid. This will give you a 1000 mg/L Mg standard solution that is suitable for use in ICP-OES. It's important to thoroughly mix the solution and make sure it is homogeneous before use.

The problem with Mg nitrate is that the water of hydration may be variable. There are pure sources of Mg carbonate that you can use.