A chemistry teacher has $670 \mathrm{~g}$ of a substance and he wants to separate the substance into 3-0z jars. How many 3-0z jars can he completely fill? The chemistry teacher can completely fill $\square$ jars.

Step 1 ：First, we need to convert the weight of the substance from grams to ounces since the jars' capacity is given in ounces. We know that 1 ounce is approximately 28.3495 grams. So, we have \(670 \, \text{g} \div 28.3495 \, \text{g/oz} \approx 23.633573784370096 \, \text{oz}\).

Step 2 ：Next, we divide the total weight in ounces by the capacity of each jar to find out how many jars can be completely filled. So, we have \(23.633573784370096 \, \text{oz} \div 3 \, \text{oz/jar} = 7.877857928123365 \, \text{jars}\).

Step 3 ：Since we can't have a fraction of a jar, we round down to the nearest whole number. So, the chemistry teacher can completely fill \(\boxed{7}\) jars.