Standard Cost of Chemical Mix:

The standard cost of a chemical mix is determined by the quantity and cost of the materials used in the mix, as well as the expected yield or output of the mix. In this case, the standard cost is calculated based on the cost of material A and material B, along with the expected yield of the mix.

Materials Used:
- 4 tons of material A
- 8 tons of material B

Cost of Materials:
- Material A: `16 per ton
- Material B: `18 per ton

Standard Yield:
The standard yield is defined as the expected output or production quantity of the mix. In this case, the standard yield is 80% of the input. This means that for every 100 units of input, the expected output is 80 units.

Calculation of Standard Cost:
To calculate the standard cost of the chemical mix, we need to consider the quantity and cost of each material, as well as the expected yield. The steps to calculate the standard cost are as follows:

1. Calculate the cost of material A:
- Quantity: 4 tons
- Cost per ton: `16
- Total cost of material A: 4 tons x `16 per ton = `64

2. Calculate the cost of material B:
- Quantity: 8 tons
- Cost per ton: `18
- Total cost of material B: 8 tons x `18 per ton = `144

3. Calculate the total cost of materials used:
- Total cost of materials = Cost of material A + Cost of material B
- Total cost of materials = `64 + `144 = `208

4. Calculate the standard yield:
- Input quantity: 4 tons + 8 tons = 12 tons
- Expected output quantity: 80% of 12 tons = 9.6 tons

5. Calculate the standard cost per unit of output:
- Standard cost per unit = Total cost of materials / Expected output quantity
- Standard cost per unit = `208 / 9.6 tons = `21.67 per ton

Therefore, the standard cost of the chemical mix is `21.67 per ton of output.

In conclusion, the standard cost of a chemical mix is determined by the quantity and cost of the materials used, as well as the expected yield. By calculating the cost of each material, totaling the cost of all materials, and considering the expected output quantity, we can determine the standard cost per unit of output. In this case, the standard cost of the chemical mix is `21.67 per ton.