How to convert w/w% weight concentration to ppm

In this post, we describe how to convert w/w% (weight concentration of a solution) to ppm (parts per million, or mg/l), with applications to liquid fertilizers.

This method is an approximation for dilute solutions.

We will focus on converting 0.8 w/w% to ppm, with 2ml of fertilizer in a 5 gallon container. This is based on Tropica fertilizer Potassium concentration. We will then verify it with the Rotala Butterfly calculator (answer is 0.845 ppm).

1) 0.8 w/w% = 8g per 1000ml

Here we have made the assumption that 1000ml of solution weighs 1000mg, which is valid for dilute solutions.

2) This is equivalent to: 0.016g per 2ml (in the fertilizer).

3) Now after pouring the fertilizer into the 5 gallon container, the concentration is 16mg per 5 gal = 16mg per 18.9271 liters.

Here we have made the approximation that the amount of fertilizer (2ml) is negligible compared to the volume of the container (5 gal).

4) This is equivalent to 0.845 mg per l or 0.845 ppm as calculated by the Rotala Butterfly calculator!

Rotala Butterfly Calculator Formula

Let a be the w/w% concentration.

Let b be the amount of fertilizer to use (in ml).


ppm = (10ab)/18.9271

In the example above,

a=0.8, b=2, so

ppm = (16)/18.9271=0.845

Drawing Chemistry Diagrams in LaTeX

The web application “mol2chemfig” is very amazing. (

Especially the search function, where one can just type the name of the chemical and out comes the chemfig code where it can be pasted to LaTeX. The mol2chemfig package needs to be imported in LaTeX before the code can be compiled. Alternatively, one can just save the PDF image (vector graphics) generated by the web app.

Below is a screenshot of what is happening:

Stolen A-Level scripts: 75 of 238 affected students will re-sit Chemistry Paper 3

Sincerely wishing good luck to those retaking their exams. It is certainly not easy, especially for boys who by that time will be in National Service. Also, for a fact-intensive course like Chemistry, with each passing month it gets harder since one’s memory would not be as fresh.


SINGAPORE: Of the 238 students whose A-Level Chemistry scripts were stolen in the United Kingdom last year, 75 have opted to be re-examined for the affected paper, the Singapore Examinations and Assessment Board (SEAB) told Channel NewsAsia on Monday (Mar 12).

The students, who received their A-Level results on Feb 23, are from Anderson JC, Anglo-Chinese JC (ACJC), Hwa Chong Institution (HCI) and Nanyang JC (NYJC). The stolen scripts were for Paper 3, which consists of free-response questions and carries a weightage of 35 per cent of the entire H2 Chemistry examination.

The final A-Level grades of the affected students were derived through a projection. But given the “unique circumstances”, the affected students were given the option to re-sit the H2 Chemistry Paper 3 in April or November, and have the better of their two grades recorded in their A-Level result slip and certificate.

Those who wished to register for the re-examination had to do so by Friday (Mar 9).

SEAB said that of the 75, there were 28 students each from AJC and ACJC, seven from HCI and 12 from NYJC.

Out of the total, 57 of the candidates registered for the re-examination in April, while 18 registered for the re-examination in November


238 Singaporean A-level H2 Chemistry exam scripts stolen in the UK

Unbelievable. I think this should be the first time such an incident happen. Hope the affected students will get a fair grade despite this unfortunate incident.

Source: Yahoo

A parcel containing 238 scripts for the 2017 GCE A-Level H2 Chemistry Paper 3 was stolen from a courier in the United Kingdom, the Singapore Examinations & Assessment Board (SEAB) said Friday (23 February).

Students from four junior colleges – Anderson JC (58 scripts), Anglo-Chinese JC (60 scripts), Hwa Chong Institution (60 scripts) and Nanyang JC (60 scripts) – were affected, SEAB said in a statement. They comprised 3 per cent of the 8,843 school candidates who sat for the paper.

The theft occurred on 16 November 2017 while the parcel was in transit from Cambridge Assessment to the examiner. The case is under UK police investigations and the scripts have not been recovered.